DOC: Fix format of categorical.rst
Our documentation should follow the PEP-8 standard, but for historical reasons there are many formats that don't. We should fix them now.

This issue is to fix the format of `doc/source/categorical.rst`. To find the errors, first is required to install a newer version of `flake8-rst`:
```
conda install -c conda-forge flake8-rst=0.5.0
```

Then, all the errors can be obtained with:
```
$ flake8-rst doc/source/categorical.rst
doc/source/categorical.rst:63:23: E231 missing whitespace after ','
doc/source/categorical.rst:63:27: E231 missing whitespace after ','
doc/source/categorical.rst:63:31: E231 missing whitespace after ','
doc/source/categorical.rst:70:27: E231 missing whitespace after ':'
doc/source/categorical.rst:70:32: E231 missing whitespace after ','
doc/source/categorical.rst:70:36: E231 missing whitespace after ','
doc/source/categorical.rst:70:40: E231 missing whitespace after ','
doc/source/categorical.rst:89:34: E231 missing whitespace after ','
doc/source/categorical.rst:89:38: E231 missing whitespace after ','
doc/source/categorical.rst:89:42: E231 missing whitespace after ','
doc/source/categorical.rst:89:64: E231 missing whitespace after ','
doc/source/categorical.rst:89:68: E231 missing whitespace after ','
doc/source/categorical.rst:93:27: E231 missing whitespace after ':'
doc/source/categorical.rst:93:32: E231 missing whitespace after ','
doc/source/categorical.rst:93:36: E231 missing whitespace after ','
doc/source/categorical.rst:93:40: E231 missing whitespace after ','
doc/source/categorical.rst:157:5: E402 module level import not at top of file
doc/source/categorical.rst:189:35: E231 missing whitespace after ','
doc/source/categorical.rst:189:49: E231 missing whitespace after ','
doc/source/categorical.rst:190:84: E501 line too long (80 > 79 characters)
doc/source/categorical.rst:201:23: E231 missing whitespace after ','
doc/source/categorical.rst:201:27: E231 missing whitespace after ','
doc/source/categorical.rst:201:31: E231 missing whitespace after ','
doc/source/categorical.rst:238:4: E402 module level import not at top of file
doc/source/categorical.rst:271:32: E241 multiple spaces after ','
doc/source/categorical.rst:296:29: E231 missing whitespace after ':'
doc/source/categorical.rst:296:38: E231 missing whitespace after ':'
doc/source/categorical.rst:312:23: E231 missing whitespace after ','
doc/source/categorical.rst:312:27: E231 missing whitespace after ','
doc/source/categorical.rst:312:31: E231 missing whitespace after ','
doc/source/categorical.rst:320:38: E231 missing whitespace after ','
doc/source/categorical.rst:320:42: E231 missing whitespace after ','
doc/source/categorical.rst:320:46: E231 missing whitespace after ','
doc/source/categorical.rst:320:68: E231 missing whitespace after ','
doc/source/categorical.rst:320:72: E231 missing whitespace after ','
doc/source/categorical.rst:357:23: E231 missing whitespace after ','
doc/source/categorical.rst:357:27: E231 missing whitespace after ','
doc/source/categorical.rst:357:31: E231 missing whitespace after ','
doc/source/categorical.rst:361:35: E231 missing whitespace after ','
doc/source/categorical.rst:361:37: E231 missing whitespace after ','
doc/source/categorical.rst:381:30: E231 missing whitespace after ','
doc/source/categorical.rst:381:32: E231 missing whitespace after ','
doc/source/categorical.rst:390:30: E231 missing whitespace after ','
doc/source/categorical.rst:390:32: E231 missing whitespace after ','
doc/source/categorical.rst:425:38: E231 missing whitespace after ','
doc/source/categorical.rst:425:42: E231 missing whitespace after ','
doc/source/categorical.rst:425:64: E231 missing whitespace after ','
doc/source/categorical.rst:425:68: E231 missing whitespace after ','
doc/source/categorical.rst:425:72: E231 missing whitespace after ','
doc/source/categorical.rst:439:25: E231 missing whitespace after ','
doc/source/categorical.rst:439:31: E231 missing whitespace after ','
doc/source/categorical.rst:441:36: E231 missing whitespace after ','
doc/source/categorical.rst:441:42: E231 missing whitespace after ','
doc/source/categorical.rst:441:50: E231 missing whitespace after ','
doc/source/categorical.rst:459:38: E231 missing whitespace after ','
doc/source/categorical.rst:459:42: E231 missing whitespace after ','
doc/source/categorical.rst:459:46: E231 missing whitespace after ','
doc/source/categorical.rst:461:23: E231 missing whitespace after ','
doc/source/categorical.rst:461:27: E231 missing whitespace after ','
doc/source/categorical.rst:461:31: E231 missing whitespace after ','
doc/source/categorical.rst:481:21: E231 missing whitespace after ','
doc/source/categorical.rst:481:23: E231 missing whitespace after ','
doc/source/categorical.rst:481:25: E231 missing whitespace after ','
doc/source/categorical.rst:482:32: E231 missing whitespace after ','
doc/source/categorical.rst:482:34: E231 missing whitespace after ','
doc/source/categorical.rst:499:21: E231 missing whitespace after ','
doc/source/categorical.rst:499:23: E231 missing whitespace after ','
doc/source/categorical.rst:499:25: E231 missing whitespace after ','
doc/source/categorical.rst:500:36: E231 missing whitespace after ','
doc/source/categorical.rst:500:38: E231 missing whitespace after ','
doc/source/categorical.rst:529:27: E203 whitespace before ':'
doc/source/categorical.rst:529:78: E231 missing whitespace after ','
doc/source/categorical.rst:529:82: E231 missing whitespace after ','
doc/source/categorical.rst:529:83: E501 line too long (99 > 79 characters)
doc/source/categorical.rst:530:27: E203 whitespace before ':'
doc/source/categorical.rst:530:32: E231 missing whitespace after ','
doc/source/categorical.rst:530:34: E231 missing whitespace after ','
doc/source/categorical.rst:530:36: E231 missing whitespace after ','
doc/source/categorical.rst:530:38: E231 missing whitespace after ','
doc/source/categorical.rst:530:40: E231 missing whitespace after ','
doc/source/categorical.rst:530:42: E231 missing whitespace after ','
doc/source/categorical.rst:530:44: E231 missing whitespace after ','
doc/source/categorical.rst:530:47: E202 whitespace before '}'
doc/source/categorical.rst:537:51: E231 missing whitespace after ','
doc/source/categorical.rst:537:55: E231 missing whitespace after ','
doc/source/categorical.rst:538:27: E231 missing whitespace after ','
doc/source/categorical.rst:563:23: E231 missing whitespace after ','
doc/source/categorical.rst:563:25: E231 missing whitespace after ','
doc/source/categorical.rst:566:28: E231 missing whitespace after ','
doc/source/categorical.rst:566:30: E231 missing whitespace after ','
doc/source/categorical.rst:569:29: E231 missing whitespace after ','
doc/source/categorical.rst:569:31: E231 missing whitespace after ','
doc/source/categorical.rst:589:23: E231 missing whitespace after ','
doc/source/categorical.rst:589:25: E231 missing whitespace after ','
doc/source/categorical.rst:599:10: E111 indentation is not a multiple of four
doc/source/categorical.rst:607:23: E231 missing whitespace after ','
doc/source/categorical.rst:607:25: E231 missing whitespace after ','
doc/source/categorical.rst:612:10: E111 indentation is not a multiple of four
doc/source/categorical.rst:635:38: E231 missing whitespace after ','
doc/source/categorical.rst:635:42: E231 missing whitespace after ','
doc/source/categorical.rst:635:46: E231 missing whitespace after ','
doc/source/categorical.rst:635:68: E231 missing whitespace after ','
doc/source/categorical.rst:635:72: E231 missing whitespace after ','
doc/source/categorical.rst:635:76: E231 missing whitespace after ','
doc/source/categorical.rst:642:31: E231 missing whitespace after ','
doc/source/categorical.rst:642:35: E231 missing whitespace after ','
doc/source/categorical.rst:642:39: E231 missing whitespace after ','
doc/source/categorical.rst:642:43: E231 missing whitespace after ','
doc/source/categorical.rst:642:47: E231 missing whitespace after ','
doc/source/categorical.rst:642:51: E231 missing whitespace after ','
doc/source/categorical.rst:642:73: E231 missing whitespace after ','
doc/source/categorical.rst:642:77: E231 missing whitespace after ','
doc/source/categorical.rst:642:81: E231 missing whitespace after ','
doc/source/categorical.rst:642:84: E501 line too long (82 > 79 characters)
doc/source/categorical.rst:643:30: E231 missing whitespace after ':'
doc/source/categorical.rst:643:35: E231 missing whitespace after ','
doc/source/categorical.rst:643:44: E231 missing whitespace after ':'
doc/source/categorical.rst:643:47: E231 missing whitespace after ','
doc/source/categorical.rst:643:49: E231 missing whitespace after ','
doc/source/categorical.rst:643:51: E231 missing whitespace after ','
doc/source/categorical.rst:643:53: E231 missing whitespace after ','
doc/source/categorical.rst:643:55: E231 missing whitespace after ','
doc/source/categorical.rst:643:57: E231 missing whitespace after ','
doc/source/categorical.rst:646:32: E231 missing whitespace after ','
doc/source/categorical.rst:646:36: E231 missing whitespace after ','
doc/source/categorical.rst:646:40: E231 missing whitespace after ','
doc/source/categorical.rst:646:62: E231 missing whitespace after ','
doc/source/categorical.rst:646:66: E231 missing whitespace after ','
doc/source/categorical.rst:647:31: E231 missing whitespace after ':'
doc/source/categorical.rst:647:37: E231 missing whitespace after ','
doc/source/categorical.rst:647:41: E231 missing whitespace after ':'
doc/source/categorical.rst:647:46: E231 missing whitespace after ','
doc/source/categorical.rst:647:50: E231 missing whitespace after ','
doc/source/categorical.rst:647:54: E231 missing whitespace after ','
doc/source/categorical.rst:647:69: E231 missing whitespace after ':'
doc/source/categorical.rst:647:72: E231 missing whitespace after ','
doc/source/categorical.rst:647:74: E231 missing whitespace after ','
doc/source/categorical.rst:647:76: E231 missing whitespace after ','
doc/source/categorical.rst:648:24: E231 missing whitespace after ','
doc/source/categorical.rst:655:34: E231 missing whitespace after ','
doc/source/categorical.rst:655:38: E231 missing whitespace after ','
doc/source/categorical.rst:655:42: E231 missing whitespace after ','
doc/source/categorical.rst:655:64: E231 missing whitespace after ','
doc/source/categorical.rst:655:68: E231 missing whitespace after ','
doc/source/categorical.rst:656:27: E231 missing whitespace after ':'
doc/source/categorical.rst:656:35: E231 missing whitespace after ','
doc/source/categorical.rst:656:39: E231 missing whitespace after ':'
doc/source/categorical.rst:656:44: E231 missing whitespace after ','
doc/source/categorical.rst:656:48: E231 missing whitespace after ','
doc/source/categorical.rst:656:52: E231 missing whitespace after ','
doc/source/categorical.rst:656:67: E231 missing whitespace after ':'
doc/source/categorical.rst:656:70: E231 missing whitespace after ','
doc/source/categorical.rst:656:72: E231 missing whitespace after ','
doc/source/categorical.rst:656:74: E231 missing whitespace after ','
doc/source/categorical.rst:674:24: E231 missing whitespace after ','
doc/source/categorical.rst:674:28: E231 missing whitespace after ','
doc/source/categorical.rst:674:32: E231 missing whitespace after ','
doc/source/categorical.rst:674:36: E231 missing whitespace after ','
doc/source/categorical.rst:674:40: E231 missing whitespace after ','
doc/source/categorical.rst:674:44: E231 missing whitespace after ','
doc/source/categorical.rst:674:48: E231 missing whitespace after ','
doc/source/categorical.rst:675:26: E231 missing whitespace after ','
doc/source/categorical.rst:675:30: E231 missing whitespace after ','
doc/source/categorical.rst:675:34: E231 missing whitespace after ','
doc/source/categorical.rst:675:38: E231 missing whitespace after ','
doc/source/categorical.rst:675:42: E231 missing whitespace after ','
doc/source/categorical.rst:675:46: E231 missing whitespace after ','
doc/source/categorical.rst:676:11: E225 missing whitespace around operator
doc/source/categorical.rst:676:15: E231 missing whitespace after ','
doc/source/categorical.rst:676:17: E231 missing whitespace after ','
doc/source/categorical.rst:676:19: E231 missing whitespace after ','
doc/source/categorical.rst:676:21: E231 missing whitespace after ','
doc/source/categorical.rst:676:23: E231 missing whitespace after ','
doc/source/categorical.rst:676:25: E231 missing whitespace after ','
doc/source/categorical.rst:677:30: E231 missing whitespace after ':'
doc/source/categorical.rst:677:35: E231 missing whitespace after ','
doc/source/categorical.rst:677:44: E231 missing whitespace after ':'
doc/source/categorical.rst:678:16: E231 missing whitespace after ','
doc/source/categorical.rst:679:16: E231 missing whitespace after ','
doc/source/categorical.rst:680:19: E231 missing whitespace after ','
doc/source/categorical.rst:696:13: E231 missing whitespace after ','
doc/source/categorical.rst:697:37: E231 missing whitespace after ','
doc/source/categorical.rst:697:41: E231 missing whitespace after ','
doc/source/categorical.rst:698:14: E231 missing whitespace after ','
doc/source/categorical.rst:698:22: E261 at least two spaces before inline comment
doc/source/categorical.rst:709:17: E231 missing whitespace after ','
doc/source/categorical.rst:763:24: E231 missing whitespace after ','
doc/source/categorical.rst:763:28: E231 missing whitespace after ','
doc/source/categorical.rst:763:32: E231 missing whitespace after ','
doc/source/categorical.rst:763:36: E231 missing whitespace after ','
doc/source/categorical.rst:763:40: E231 missing whitespace after ','
doc/source/categorical.rst:763:44: E231 missing whitespace after ','
doc/source/categorical.rst:764:31: E231 missing whitespace after ','
doc/source/categorical.rst:764:35: E231 missing whitespace after ','
doc/source/categorical.rst:764:39: E231 missing whitespace after ','
doc/source/categorical.rst:764:43: E231 missing whitespace after ','
doc/source/categorical.rst:764:47: E231 missing whitespace after ','
doc/source/categorical.rst:764:51: E231 missing whitespace after ','
doc/source/categorical.rst:764:73: E231 missing whitespace after ','
doc/source/categorical.rst:765:16: E231 missing whitespace after ','
doc/source/categorical.rst:765:18: E231 missing whitespace after ','
doc/source/categorical.rst:765:20: E231 missing whitespace after ','
doc/source/categorical.rst:765:22: E231 missing whitespace after ','
doc/source/categorical.rst:765:24: E231 missing whitespace after ','
doc/source/categorical.rst:765:26: E231 missing whitespace after ','
doc/source/categorical.rst:766:30: E231 missing whitespace after ':'
doc/source/categorical.rst:766:35: E231 missing whitespace after ','
doc/source/categorical.rst:766:44: E231 missing whitespace after ':'
doc/source/categorical.rst:768:16: E231 missing whitespace after ','
doc/source/categorical.rst:768:27: E231 missing whitespace after ','
doc/source/categorical.rst:768:30: E231 missing whitespace after ','
doc/source/categorical.rst:768:35: E231 missing whitespace after ','
doc/source/categorical.rst:771:20: E231 missing whitespace after ','
doc/source/categorical.rst:771:31: E231 missing whitespace after ','
doc/source/categorical.rst:771:34: E231 missing whitespace after ','
doc/source/categorical.rst:771:39: E231 missing whitespace after ','
doc/source/categorical.rst:779:19: E231 missing whitespace after ','
doc/source/categorical.rst:779:49: E231 missing whitespace after ','
doc/source/categorical.rst:779:71: E231 missing whitespace after ','
doc/source/categorical.rst:782:23: E231 missing whitespace after ','
doc/source/categorical.rst:782:53: E231 missing whitespace after ','
doc/source/categorical.rst:782:75: E231 missing whitespace after ','
doc/source/categorical.rst:782:79: E231 missing whitespace after ','
doc/source/categorical.rst:782:84: E501 line too long (80 > 79 characters)
doc/source/categorical.rst:790:27: E231 missing whitespace after ':'
doc/source/categorical.rst:790:30: E231 missing whitespace after ','
doc/source/categorical.rst:790:32: E231 missing whitespace after ','
doc/source/categorical.rst:790:34: E231 missing whitespace after ','
doc/source/categorical.rst:790:36: E231 missing whitespace after ','
doc/source/categorical.rst:790:44: E231 missing whitespace after ':'
doc/source/categorical.rst:790:49: E231 missing whitespace after ','
doc/source/categorical.rst:790:53: E231 missing whitespace after ','
doc/source/categorical.rst:790:57: E231 missing whitespace after ','
doc/source/categorical.rst:790:61: E231 missing whitespace after ','
doc/source/categorical.rst:791:15: E231 missing whitespace after ','
doc/source/categorical.rst:791:42: E231 missing whitespace after ','
doc/source/categorical.rst:791:64: E231 missing whitespace after ','
doc/source/categorical.rst:792:15: E231 missing whitespace after ','
doc/source/categorical.rst:792:42: E231 missing whitespace after ','
doc/source/categorical.rst:792:64: E231 missing whitespace after ','
doc/source/categorical.rst:806:25: E231 missing whitespace after ','
doc/source/categorical.rst:807:14: E231 missing whitespace after ','
doc/source/categorical.rst:808:30: E231 missing whitespace after ':'
doc/source/categorical.rst:808:42: E231 missing whitespace after ':'
doc/source/categorical.rst:809:24: E231 missing whitespace after ','
doc/source/categorical.rst:818:47: E231 missing whitespace after ','
doc/source/categorical.rst:820:22: E231 missing whitespace after ','
doc/source/categorical.rst:843:5: E402 module level import not at top of file
doc/source/categorical.rst:979:5: E402 module level import not at top of file
doc/source/categorical.rst:988:30: E231 missing whitespace after ':'
doc/source/categorical.rst:988:40: E231 missing whitespace after ':'
doc/source/categorical.rst:988:43: E231 missing whitespace after ','
doc/source/categorical.rst:988:45: E231 missing whitespace after ','
doc/source/categorical.rst:988:47: E231 missing whitespace after ','
doc/source/categorical.rst:988:49: E231 missing whitespace after ','
doc/source/categorical.rst:988:51: E231 missing whitespace after ','
doc/source/categorical.rst:996:84: E501 line too long (82 > 79 characters)
doc/source/categorical.rst:1065:24: E231 missing whitespace after ','
doc/source/categorical.rst:1065:31: E226 missing whitespace around arithmetic operator
doc/source/categorical.rst:1108:10: E111 indentation is not a multiple of four
doc/source/categorical.rst:1129:36: E231 missing whitespace after ','
doc/source/categorical.rst:1129:38: E231 missing whitespace after ','
doc/source/categorical.rst:1129:40: E231 missing whitespace after ','
doc/source/categorical.rst:1132:9: E265 block comment should start with '# '
doc/source/categorical.rst:1134:10: E111 indentation is not a multiple of four
doc/source/categorical.rst:1148:27: E231 missing whitespace after ':'
doc/source/categorical.rst:1148:30: E231 missing whitespace after ','
doc/source/categorical.rst:1148:32: E231 missing whitespace after ','
doc/source/categorical.rst:1148:34: E231 missing whitespace after ','
doc/source/categorical.rst:1149:27: E231 missing whitespace after ':'
doc/source/categorical.rst:1149:32: E231 missing whitespace after ','
doc/source/categorical.rst:1149:36: E231 missing whitespace after ','
doc/source/categorical.rst:1149:40: E231 missing whitespace after ','
doc/source/categorical.rst:1150:30: E231 missing whitespace after ':'
doc/source/categorical.rst:1150:48: E231 missing whitespace after ','
doc/source/categorical.rst:1150:50: E231 missing whitespace after ','
doc/source/categorical.rst:1150:52: E231 missing whitespace after ','
doc/source/categorical.rst:1167:29: E231 missing whitespace after ','
doc/source/categorical.rst:1167:31: E231 missing whitespace after ','
doc/source/categorical.rst:1167:33: E231 missing whitespace after ','
doc/source/categorical.rst:1167:51: E231 missing whitespace after ','
doc/source/categorical.rst:1167:53: E231 missing whitespace after ','
doc/source/categorical.rst:1167:55: E231 missing whitespace after ','
doc/source/categorical.rst:1168:19: E231 missing whitespace after ','
doc/source/categorical.rst:1168:23: E231 missing whitespace after ','
doc/source/categorical.rst:1168:27: E231 missing whitespace after ','
doc/source/categorical.rst:1169:16: E231 missing whitespace after ','
doc/source/categorical.rst:1169:18: E231 missing whitespace after ','
doc/source/categorical.rst:1169:20: E231 missing whitespace after ','
doc/source/categorical.rst:1170:33: E231 missing whitespace after ':'
doc/source/categorical.rst:1170:51: E231 missing whitespace after ':'
doc/source/categorical.rst:1184:28: E231 missing whitespace after ','
doc/source/categorical.rst:1184:30: E231 missing whitespace after ','
doc/source/categorical.rst:1184:32: E231 missing whitespace after ','
doc/source/categorical.rst:1184:51: E231 missing whitespace after ','
doc/source/categorical.rst:1184:53: E231 missing whitespace after ','
doc/source/categorical.rst:1184:55: E231 missing whitespace after ','
doc/source/categorical.rst:1184:57: E231 missing whitespace after ','
doc/source/categorical.rst:1190:34: E231 missing whitespace after ','
doc/source/categorical.rst:1190:36: E231 missing whitespace after ','
doc/source/categorical.rst:1190:38: E231 missing whitespace after ','
doc/source/categorical.rst:1190:40: E231 missing whitespace after ','
doc/source/categorical.rst:1197:28: E231 missing whitespace after ','
doc/source/categorical.rst:1197:30: E231 missing whitespace after ','
doc/source/categorical.rst:1197:32: E231 missing whitespace after ','
doc/source/categorical.rst:1197:51: E231 missing whitespace after ','
doc/source/categorical.rst:1197:53: E231 missing whitespace after ','
doc/source/categorical.rst:1197:55: E231 missing whitespace after ','
doc/source/categorical.rst:1197:57: E231 missing whitespace after ','
```

After fixing this issue, the previous command should not report any error.