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

closes #7870
@jreback what do you think about this? it breaks functionality like:

```python
df.groupby('c').filter(lambda x: x.mean() < 10)
```

instead you'd have to be more explicit and do

```python
df.groupby('c').filter(lambda x: x.mean().item() < 10)
```

or

```python
df.groupby('c').filter(lambda x: (x.mean() < 10).item())
```