BUG/FIX: groupby should raise on multi-valued filter

closes #7870
@cpcloud ok, so you are +1 on allowing single element sequences, then. ok, I would agree with that. That's the bug here, right? I think if ``.item()`` its a len-1 sequence (and you just need to apply ``.item()``) then its ok to take that as a result.

unless I am misunderstanding you. I think thats a nice convience