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.
@WillAyd, Thanks. There are a few places where they actually are labels, and I even found one place where it started out as "real" labels and later changed into codes, while still being called labels...not easy to keep track of.

I thinkt we should try to get the label names that actually are code arrays to be renamed to codes. For example in ``pd.factorize`` the labels name is still used. Is this ok to take in, @WillAyd ? looks good. can you merge master and ping on green (we just pushed some groupby changes).