Clean up Abstract and Naming Definitions for GroupBy

[WillAyd]

Summary of changes:

• Made _iterate_slices abstract in base class, moved pre-existing definition from Base -> Series where it was actually being called (DataFrame already overrides)
• Made _wrap_transformed_output Abstract in base class, as both Series and DataFrame override this
• Redefined _wrap_applied_output; this was Abstract in base class before but its definition was not in sync with how the subclasses actually defined
• Renamed _wrap_output to _wrap_series_output as it is only valid for SeriesGroupBy
• Renamed _wrap_generic_output to _wrap_frame_output as it is only valid for DataFrameGroupBy
• Renamed _aggregate_generic to _aggregate_frame as it is only valid for DataFrameGroupBy

More to come in separate PRs

[pep8speaks]

Hello @WillAyd! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

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

Comment last updated at 2019-10-08 16:15:58 UTC

[jbrockmendel]

makes sense. LGTM pending green

[jreback]

thanks @WillAyd

doc-strings would be very helpful on some of these as well :->

[WillAyd]

Agreed - docstrings and annotations should come in one of these