BUG: Can't assign values via .ix or .loc to a Panel with MultiIndex
If you have a Panel with a MultiIndex, you can read but not write to it with `.ix` (same with `.loc`).

Before 0.13 I would normally do something like `pn[item].ix[a, b][stuff] = values`, surprisingly it used to work (now it spits out a `SettingWithCopyWarning` but still kinda works).

So what are the valid options in 0.13?

``` python
>>> df = pd.DataFrame([1] * 3, pd.MultiIndex.from_tuples(zip(range(3), range(3))))
>>> df
     0
0 0  1
1 1  1
2 2  1

[3 rows x 1 columns]

>>> pn = pd.Panel(dict((k, df.copy() * k) for k in xrange(2)))
>>> pn
<class 'pandas.core.panel.Panel'>
Dimensions: 2 (items) x 3 (major_axis) x 1 (minor_axis)
Items axis: 0 to 1
Major_axis axis: (0, 0) to (2, 2)
Minor_axis axis: 0 to 0

>>> pn.ix[1, (0, 0), 0]
1

>>> pn.ix[1, (0, 0), 0] = 1
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-64-f48c3a7f93c3> in <module>()
----> 1 pn.ix[1, (0, 0), 0] = 1

/home/smirnov/anaconda/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/indexing.pyc in __setitem__(self, key, value)
     96             indexer = self._convert_to_indexer(key, is_setter=True)
     97
---> 98         self._setitem_with_indexer(indexer, value)
     99
    100     def _has_valid_type(self, k, axis):

/home/smirnov/anaconda/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/indexing.pyc in _setitem_with_indexer(self, indexer, value)
    299                 obj = self.obj[item]
    300                 index = obj.index
--> 301                 idx = indexer[:info_axis][0]
    302                 try:
    303                     if idx in index:

IndexError: tuple index out of range
> /home/smirnov/anaconda/lib/python2.7/site-packages/pandas-0.13.1-py2.7-linux-x86_64.egg/pandas/core/indexing.py(301)_setitem_with_indexer()
    300                 index = obj.index
--> 301                 idx = indexer[:info_axis][0]
    302                 try:
```
