ENH: Add group-wise checks

[TomAugspurger]

All our checks work around DataFrames right now. I had a case today for checking that a groupwise minimum was above a threshold.

APIwise, do we mirror the current API in a grouped module, that expects DataFrameGroupbys and returns DataFrames, or do we expand the function signature of each check/decorator to accept a by?

[drorata]

So you want to add a defensive test on a DataFrameGroupby object?