api for timeseries interval pivoting
 For example, want easy to and from timeseries for following data.

```
Out[179]:
      Jan  Feb  Mar  Apr  May  Jun  Jul  Aug  Sep  Oct  Nov  Dec
1949  112  118  132  129  121  135  148  148  136  119  104  118
1950  115  126  141  135  125  149  170  170  158  133  114  140
1951  145  150  178  163  172  178  199  199  184  162  146  166
1952  171  180  193  181  183  218  230  242  209  191  172  194
1953  196  196  236  235  229  243  264  272  237  211  180  201
1954  204  188  235  227  234  264  302  293  259  229  203  229
1955  242  233  267  269  270  315  364  347  312  274  237  278
1956  284  277  317  313  318  374  413  405  355  306  271  306
1957  315  301  356  348  355  422  465  467  404  347  305  336
1958  340  318  362  348  363  435  491  505  404  359  310  337
1959  360  342  406  396  420  472  548  559  463  407  362  405
1960  417  391  419  461  472  535  622  606  508  461  390  432
```

One idea is something like

```
df.stack().reindex_periods(year=0, month=1, convention='begin')
```

Or maybe API functions like:

```
consolidate_time_index(df.stack(), year=0, month=1, convention='begin')
```

And the reverse operation:

```
expand_time_index(series, year=0, month=1)
```
