ValueError: Cannot index with multidimensional key
```
items = ['A','B']
major_axis=['a','b','c']
minor_axis=[1,2,3]
pl = pd.Panel(np.random.randn(2,3,3), items=items, major_axis=major_axis, minor_axis=minor_axis)
cond = pl['A'] > 1
```

can we possibly support the following reference selection (not a copy)?
``pl.loc['A', cond] = ...``

if not, is there a fast way to solve this? 

Thank you:-)
