text: Title: [ENH] Method for adding functionality to GroupBy
  Body: It would be nice to be able to add functionality to the Pandas `GroupBy` objects: `GroupBy`, `DataFrameGroupBy`, `SeriesGroupBy`. There's no convenient accessor interface to do this, but maybe there's a way to reliably monkeypatch them. This would allow us to create nifty aggregation / apply functions and avoid the `.groupby(...).apply()` route for tasks we may encounter routinely. It could also potentially open up opportunities to speed up such operations... `.groupby().apply()` can often be slow for large numbers of groups.