DEPR: deprecate .ix in favor of .loc/.iloc
closes #14218 
closes #15116 

This shows a pretty big deprecation message, though its instructive.
```
In [1]: df = pd.DataFrame({'A': [1, 2, 3],
   ...:                        'B': [4, 5, 6]},
   ...:                       index=list('abc'))
   ...: df
   ...: 
Out[1]: 
   A  B
a  1  4
b  2  5
c  3  6

In [2]: df.ix[[0, 2], 'A']
/Users/jreback/miniconda3/envs/pandas/bin/ipython:1: FutureWarning: 
.ix is deprecated. Please use
.loc for label based indexing or
.iloc for positional indexing

You an do multi-axes indexing like the following
>>> df = pd.DataFrame({'A': [1, 2, 3],
                       'B': [4, 5, 6]},
                      index=list('abc'))
>>> df
   A  B
a  1  4
b  2  5
c  3  6

From prior versions
>>> df.ix[[0, 2], 'A']
a    1
c    3
Name: A, dtype: int64

Using .loc
>>> df.loc[df.index[[0, 2]], 'A']
a    1
c    3
Name: A, dtype: int64

Using .iloc
>>> df.iloc[[0, 2]], df.columns.get_loc('A')]
a    1
c    3
Name: A, dtype: int64
```