ENH: Keep series name in GroupBy agg/apply ops

When possible, attempt to preserve the series name when performing groupby
operations.  This facilitates reshaping/indexing operations on the result of the
groupby/apply or groupby/agg operation.  Fixes GH6265 and GH6124.  Added
example to groupby.rst and description to API changes for v0.14.
Merge pull request #6068 from bburan-galenea/bburan/dataframe_groupby_apply_series_name

ENH: Keep series name when merging GroupBy result