groupby and np.dot with MultiIndex
Should the following work? On revision 07403ca.

```
weights = np.random.randn(4,4)
panel = pd.util.testing.makePanel(nper=2).to_frame()
panel.groupby(level=0).apply(lambda x : np.dot(weights, x))
```

I realize I can do

```
panel.groupby(level=0).apply(lambda x : pd.DataFrame(np.dot(weights, x), index=x.index, columns=x.columns))
```

Would it make sense to just assume that the result should be wrapped like this if it's conformable to the other level of the MultiIndex? Too much of a special case?
