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

closes #7870
you could use a try: except: with ``series.bool()`` (or something like this). I think to fix this, which deals with a 1 element boolean array (and raises otherwise); and would fail on ndarrays (or use code like that, e.g. accepting a 1-element ndarray/series of boolean dtype is ok (or a scalar).

I don't think allowing a non-reduction op is ok though (e.g. the user should be using any/all)