consolidate when dispatching DataFrame function for a DataFrameGroupBy

[machow]

in siuba.dply.verbs, lots of this repeated right now...

@unite.register(DataFrameGroupBy)
def _unite_gdf(__data, *args, **kwargs):
    # TODO: consolidate these trivial group by dispatched funcs

    groupings = __data.grouper.groupings

    df = __data.obj

    f_unite = unite.registry[pd.DataFrame]
    out = f_unite(df, *args, **kwargs)

    return out.groupby(groupings)