BUG: pivot_table with margins=True fails for categorical dtype, #10989
This is a fix for the issue reported in #10989. I suspect this is an example of "fixing the symptom" rather than "fixing the problem", but I think it makes clear what the source of the problem is: to compute margins, the pivot table must add a row and/or column to the result. If the index or column is categorical, a new value cannot be added.

Let me know if you think there are better approaches to this.

I should add one thing: I decided that rather than adding a new item to the category, it would be cleaner to simply change categories to objects rather than trying to track down all the corner cases of hierarchical indices with categorical levels.
 A much cleaner solution, IMO, would be to add a utility function that does something along the lines of "add a new entry to this index, even if it requires a new category". I imagine there are other places in the package where this sort of thing might happen, and such a utility could be used there as well.
 I think we should return a regular Index with object dtypes. What happens if the user has a `category` called 'All'? I suspect that any fix involving categories will break/be fragile (just a hunch, haven't tried).
 @TomAugspurger – I checked – it turns out this is another bug in the current codebase, even if you're not using categories! If one of the index entries is called `All`, the attempt to compute margins will overwrite it:

``` python
In [19]: data = pd.DataFrame({'x': np.arange(99),
                     'y': np.arange(99) // 50,
                     'z': np.arange(99) % 3})

In [20]: data.z = np.array(['Any', 'All', 'None'])[data.z]

In [21]: data.pivot_table('x', 'y', 'z')
Out[21]: 
z   All   Any  None
y                  
0  25.0  24.0  24.5
1  74.5  73.5  74.0

In [22]: data.pivot_table('x', 'y', 'z', margins=True)
Out[22]: 
z     All   Any  None
y                    
0    24.5  24.0  24.5
1    74.0  73.5  74.0
All  49.0  48.0  50.0
```
 @jakevdp can you update according to comments
 I think I've already addressed all comments above – any others you have in mind? Any reason you closed this without merging?
 I thought I put the comments before

this needs a more general soln as it too much if/then on type determination
 I guess I'm not entirely clear about what you're wanting as a "more general" solution. Any specific ideas?
 So there is something more going on here; this bug report is a sympton of a different issue. Namely,
that we allow a Categorical as a _level_ of a MultiIndex. But in fact we should simply convert them directly to an object dtype when creating the multi-index in the first place; these are de-facto the same.

```
In [4]: data = pd.DataFrame({'x': np.arange(8),'y': Series(np.arange(8) // 4).astype('category'),'z': Series(np.arange(8) % 2).astype('category')})

In [5]: data
Out[5]: 
   x  y  z
0  0  0  0
1  1  0  1
2  2  0  0
3  3  0  1
4  4  1  0
5  5  1  1
6  6  1  0
7  7  1  1

In [6]: data.dtypes
Out[6]: 
x       int64
y    category
z    category
dtype: object

In [7]: data.groupby(['y','z']).agg('mean')
Out[7]: 
     x
y z   
0 0  1
  1  2
1 0  5
  1  6
```

```
In [7]: data.groupby(['y','z']).agg('mean')
Out[7]: 
     x
y z   
0 0  1
  1  2
1 0  5
  1  6

In [8]: data.groupby(['y','z']).agg('mean').index.levels[0]
Out[8]: CategoricalIndex([0, 1], categories=[0, 1], ordered=False, name=u'y', dtype='category')

In [9]: data.groupby(['y','z']).agg('mean').index.levels[1]
Out[9]: CategoricalIndex([0, 1], categories=[0, 1], ordered=False, name=u'z', dtype='category')

In [10]: data.groupby(['y','z']).agg('mean').index.values   
Out[10]: array([(0, 0), (0, 1), (1, 0), (1, 1)], dtype=object)
```

So this is the _same_ as the index in [10]

```
In [15]: idx = pd.MultiIndex.from_tuples([(0, 0), (0, 1), (1, 0), (1, 1)],names=['y','z'])

In [16]: idx
Out[16]: 
MultiIndex(levels=[[0, 1], [0, 1]],
           labels=[[0, 0, 1, 1], [0, 1, 0, 1]],
           names=[u'y', u'z'])
```

So I think that `MultiIndex` creation should coerce `Categoricals` on construction. This can be done in the `MultiIndex.__init__` (keep all of these existing test), prob need a couple more.
 I suppose we should close this PR then, and leave the issue open. Hacking into the internals of MultiIndex is well beyond my level of comfort with the library.
 haha. well, I'll take your tests in any event. So going to leave open for a bit.
 Sounds good – thanks!
