REF: Move generic methods to aggregation.py
xref #29116

https://github.com/pandas-dev/pandas/pull/29116#pullrequestreview-340272465

based on @jreback comment, this PR is a precursor PR for #29116 to make PR smaller and more readable
@jreback could you take a look? pls lemme know what changes you want to see here? and then I could start finishing up #29116 after this is merged. Thanks! Hello @charlesdong1991! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:









There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-01-20 22:05:57 UTC lgtm. ping on green. thanks @charlesdong1991 