ERR: show SettingWithCopyWarning on Panel assignments if possible
This involves incorrect pickling of Pandas Panel values. From what I've seen, it occurs when new values of a different data type are assigned to a Pandas Panel axis. Everything functions well until the panel is pickled. When the pickle is read the old values appear. In the example below floats are reassigned as objects, but when the pickle is loaded back up, it contains the old float values. Also noted:
- does not happen with DataFrames
- does not happen if the new values are of the same data type as the old ones (in that case id() of the Series is the same)
- happens whenever data type is changed (in the snippet below float to str but happens in reverse too). 

``` python
import pandas as pd
import numpy as np
import pickle

wp = pd.Panel(np.random.randn(2,3,2), 
                        items=['Item1', 'Item2'],
                        major_axis=[1,2,3], 
                        minor_axis=['float', 'string'])
print wp['Item1']

wp['Item1']['string'] = 'blue'
print wp['Item1']

with open('test_pickle', 'wb') as f:
  pickle.dump(wp, f)
wp2 = pickle.load( open('test_pickle', 'rb') )
print wp2['Item1']
```

UPDATE: It's not a pickle problem. For example, a transpose operation on the Panel also erases the new values and brings back the old ones.

``` python
wp=wp.transpose(2,0,1)
wp=wp.transpose(1,2,0)
print wp['Item1']
```
