Binary operations between numeric & categorical should raise

[jcrist]

Currently this only happens if the dispatch hits categorical first:

In [21]: df = pd.DataFrame({'x': range(10), 'y': list(map(float, range(10))), 'z': list('abcde')*2})

In [22]: df.z = df.z.astype('category')

In [23]: gdf = gd.DataFrame.from_pandas(df)

In [24]: gdf.x + gdf.z
Out[24]:

 0 144396680282898688
 1               1028
 2                  7
 3                  8
 4                  9
 5                 10
 6                 11
 7                 12
 8                 13
 9                 14

In [25]: gdf.z + gdf.x
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-25-01cf7f26eb92> in <module>()
----> 1 gdf.z + gdf.x

~/Code/pygdf/pygdf/series.py in __add__(self, other)
    356
    357     def __add__(self, other):
--> 358         return self._binaryop(other, 'add')
    359
    360     def __sub__(self, other):

~/Code/pygdf/pygdf/series.py in _binaryop(self, other, fn)
    345         if not isinstance(other, Series):
    346             return NotImplemented
--> 347         return self._impl.binary_operator(fn, self, other)
    348
    349     def _unaryop(self, fn):

~/Code/pygdf/pygdf/categorical.py in binary_operator(self, binop, lhs, rhs)
     73     def binary_operator(self, binop, lhs, rhs):
     74         msg = 'Categorical cannot perform the operation: {}'.format(binop)
---> 75         raise TypeError(msg)
     76
     77     def unary_operator(self, unaryop, series):

TypeError: Categorical cannot perform the operation: add