DOC: Fix format of io.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/io.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/io.rst 
doc/source/io.rst:14:4: E402 module level import not at top of file
doc/source/io.rst:19:4: E402 module level import not at top of file
doc/source/io.rst:22:4: E402 module level import not at top of file
doc/source/io.rst:430:88: E501 line too long (84 > 79 characters)
doc/source/io.rst:481:4: E402 module level import not at top of file
doc/source/io.rst:666:17: E126 continuation line over-indented for hanging indent
doc/source/io.rst:680:10: E222 multiple spaces after operator
doc/source/io.rst:721:83: E501 line too long (95 > 79 characters)
doc/source/io.rst:815:10: E222 multiple spaces after operator
doc/source/io.rst:898:4: E402 module level import not at top of file
doc/source/io.rst:995:9: E111 indentation is not a multiple of four
doc/source/io.rst:1019:83: E501 line too long (87 > 79 characters)
doc/source/io.rst:1020:83: E501 line too long (89 > 79 characters)
doc/source/io.rst:1021:83: E501 line too long (95 > 79 characters)
doc/source/io.rst:1036:10: E222 multiple spaces after operator
doc/source/io.rst:1135:10: E222 multiple spaces after operator
doc/source/io.rst:1147:12: E222 multiple spaces after operator
doc/source/io.rst:1169:9: E225 missing whitespace around operator
doc/source/io.rst:1440:55: E231 missing whitespace after ','
doc/source/io.rst:1455:4: E402 module level import not at top of file
doc/source/io.rst:1483:4: E402 module level import not at top of file
doc/source/io.rst:1970:13: E128 continuation line under-indented for visual indent
doc/source/io.rst:1971:13: E128 continuation line under-indented for visual indent
doc/source/io.rst:2023:12: E999 SyntaxError: invalid syntax
doc/source/io.rst:2062:4: E402 module level import not at top of file
doc/source/io.rst:2075:4: E402 module level import not at top of file
doc/source/io.rst:2084:14: E127 continuation line over-indented for visual indent
doc/source/io.rst:2086:19: E126 continuation line over-indented for hanging indent
doc/source/io.rst:2088:14: E127 continuation line over-indented for visual indent
doc/source/io.rst:2089:26: E128 continuation line under-indented for visual indent
doc/source/io.rst:2090:26: E128 continuation line under-indented for visual indent
doc/source/io.rst:2091:13: E127 continuation line over-indented for visual indent
doc/source/io.rst:2094:19: E126 continuation line over-indented for hanging indent
doc/source/io.rst:2149:8: E124 closing bracket does not match visual indentation
doc/source/io.rst:2189:6: E402 module level import not at top of file
doc/source/io.rst:2325:4: E402 module level import not at top of file
doc/source/io.rst:2823:26: E231 missing whitespace after ':'
doc/source/io.rst:2823:44: E231 missing whitespace after ':'
doc/source/io.rst:2824:66: E231 missing whitespace after ','
doc/source/io.rst:2845:83: E501 line too long (80 > 79 characters)
doc/source/io.rst:2853:4: E402 module level import not at top of file
doc/source/io.rst:3175:4: E402 module level import not at top of file
doc/source/io.rst:3252:4: E402 module level import not at top of file
doc/source/io.rst:3309:41: E201 whitespace after '{'
doc/source/io.rst:3309:50: E202 whitespace before '}'
doc/source/io.rst:3424:4: E402 module level import not at top of file
doc/source/io.rst:3438:30: E231 missing whitespace after ','
doc/source/io.rst:3453:4: E402 module level import not at top of file
doc/source/io.rst:3462:28: E127 continuation line over-indented for visual indent
doc/source/io.rst:3467:28: E127 continuation line over-indented for visual indent
doc/source/io.rst:3481:13: E128 continuation line under-indented for visual indent
doc/source/io.rst:3482:13: E128 continuation line under-indented for visual indent
doc/source/io.rst:3484:37: E225 missing whitespace around operator
doc/source/io.rst:3485:12: E128 continuation line under-indented for visual indent
doc/source/io.rst:3486:12: E126 continuation line over-indented for hanging indent
doc/source/io.rst:3588:30: E241 multiple spaces after ','
doc/source/io.rst:3663:38: E231 missing whitespace after ':'
doc/source/io.rst:3670:52: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3670:54: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3823:83: E501 line too long (81 > 79 characters)
doc/source/io.rst:3865:4: E402 module level import not at top of file
doc/source/io.rst:3866:30: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3866:32: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3866:60: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3866:62: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3866:64: E201 whitespace after '['
doc/source/io.rst:3866:83: E501 line too long (140 > 79 characters)
doc/source/io.rst:3866:140: E202 whitespace before ']'
doc/source/io.rst:3950:45: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3950:47: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:3979:7: E111 indentation is not a multiple of four
doc/source/io.rst:4006:9: E111 indentation is not a multiple of four
doc/source/io.rst:4006:23: E226 missing whitespace around arithmetic operator
doc/source/io.rst:4011:9: E111 indentation is not a multiple of four
doc/source/io.rst:4108:38: E127 continuation line over-indented for visual indent
doc/source/io.rst:4113:66: E202 whitespace before '}'
doc/source/io.rst:4114:30: E127 continuation line over-indented for visual indent
doc/source/io.rst:4123:30: E127 continuation line over-indented for visual indent
doc/source/io.rst:4123:38: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:4123:40: E251 unexpected spaces around keyword / parameter equals
doc/source/io.rst:4162:46: E202 whitespace before ')'
doc/source/io.rst:4335:49: E202 whitespace before '}'
doc/source/io.rst:4349:4: E402 module level import not at top of file
doc/source/io.rst:4427:29: E127 continuation line over-indented for visual indent
doc/source/io.rst:4438:4: E402 module level import not at top of file
doc/source/io.rst:4526:4: E402 module level import not at top of file
doc/source/io.rst:4592:4: E402 module level import not at top of file
doc/source/io.rst:4676:4: E402 module level import not at top of file
doc/source/io.rst:4725:84: E501 line too long (85 > 79 characters)
doc/source/io.rst:4746:4: E402 module level import not at top of file
doc/source/io.rst:4805:4: E402 module level import not at top of file
doc/source/io.rst:4836:4: E402 module level import not at top of file
doc/source/io.rst:4838:36: E231 missing whitespace after ','
doc/source/io.rst:4838:39: E231 missing whitespace after ','
doc/source/io.rst:4839:4: E128 continuation line under-indented for visual indent
doc/source/io.rst:4839:31: E231 missing whitespace after ','
doc/source/io.rst:4839:34: E231 missing whitespace after ','
doc/source/io.rst:4840:4: E122 continuation line missing indentation or outdented
doc/source/io.rst:4840:31: E231 missing whitespace after ','
doc/source/io.rst:4840:34: E231 missing whitespace after ','
doc/source/io.rst:4842:8: E221 multiple spaces before operator
doc/source/io.rst:4873:5: E402 module level import not at top of file
doc/source/io.rst:5002:84: E501 line too long (81 > 79 characters)
doc/source/io.rst:5057:4: E402 module level import not at top of file
doc/source/io.rst:5067:8: E128 continuation line under-indented for visual indent
doc/source/io.rst:5074:65: E712 comparison to True should be 'if cond is True:' or 'if cond:'
doc/source/io.rst:5080:5: E402 module level import not at top of file
doc/source/io.rst:5242:4: E402 module level import not at top of file
```

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