Collect methods in generic/frame/series by purpose
Analogous to #23961.  Less aggressive since these classes are massive.

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/jbrockmendel/pandas/blob/5118fc330e2bf23adeab4384dbe1236ed89e7b46/pandas/core/frame.py) !

 - There are no PEP8 issues in the file [`pandas/core/generic.py`](https://github.com/jbrockmendel/pandas/blob/5118fc330e2bf23adeab4384dbe1236ed89e7b46/pandas/core/generic.py) !

 - There are no PEP8 issues in the file [`pandas/core/series.py`](https://github.com/jbrockmendel/pandas/blob/5118fc330e2bf23adeab4384dbe1236ed89e7b46/pandas/core/series.py) !

 Travis fail is unrelated flake8-rst problem thanks