Default axis doesn't match that of numpy.ufunc.reduce

[hameerabbasi]

The default axis doesn't match that of numpy.ufunc.reduce. There, it is (0,) but we use None.

Minimal example:

>>> import numpy as np
>>> import sparse
>>> x = np.eye(5)
>>> s = sparse.COO.from_numpy(x)
>>> np.add.reduce(s)
5.0
>>> np.add.reduce(x)
array([1., 1., 1., 1., 1.])