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

closes #7870
The main question is "Should we allow single element sequences in filter results?"

non reduction ops *do* raise, so there's no issue there.

Because `DataFramGroupBy._selected_obj` is a `DataFrame` (even when you group on a single column), when you do things like `df.groupby('c').filter(lambda x: x.mean() < 10)`, `x.mean()` will be the mean over a single column `DataFrame` which is of course a `Series`.

there's nothing to "fix" re single element behavior per se, just decided whether or not to break it to force users to be more explicit about things.

frankly, i think that the fact that a reduction on `_selected_obj` is a single element `Series` is an implementation detail that users shouldn't have to think about, even if it's less explicit on their side. this has the added benefit of being backwards compatible

therefore i vote :-1: on disallowing single element sequences