BUG: assignment with panels should have SettingWithCopy as appropriate
http://stackoverflow.com/questions/23756711/pandas-panel-copy-versus-view

[55] is a view, so `is_copy` should be set (and is not)

```
In [53]: p = tm.makePanel()

In [54]: df = p.iloc[0]

In [55]: df[:] = 0

In [56]: (df.values.ravel()==0).all()
Out[56]: True

In [57]: (p.iloc[0].values.ravel()==0).all()
Out[57]: True

```
