select, tranform, combine do not work on GDTable

While experimental, the documentation explicitly says
> As of today select, transform and combine are available with more operations to come in the future. You can use the exact same DataFrames.jl minilanguage when using these operations.

In DataFrames.jl minilanguage, `select`, `transform`, and `combine` work on grouped dataframes as well. It would be great to have them also working on `GDTable`.