API: rename labels to codes in core/groupby
This PR renames the various ``*label*`` names in core/groupby to like-named ``*codes*``.

I think the name ``label`` can be confused by the single values in a index, and ``codes`` sound smore like an array of ints, so by renaming we get a cleaner nomenclature, IMO.

All these attributes/methods are internal, so no deprecations needed.