select, tranform, combine do not work on GDTable #66
Description
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.