Return the sum of the array elements over the given axis. Masked elements are set to 0 internally.
Parameters :  axis : {None, 1, int}, optional
dtype : {None, dtype}, optional
out : {None, ndarray}, optional


Returns :  sum_along_axis : MaskedArray or scalar

Examples
>>> x = np.ma.array([[1,2,3],[4,5,6],[7,8,9]], mask=[0] + [1,0]*4)
>>> print x
[[1  3]
[ 5 ]
[7  9]]
>>> print x.sum()
25
>>> print x.sum(axis=1)
[4 5 16]
>>> print x.sum(axis=0)
[8 5 12]
>>> print type(x.sum(axis=0, dtype=np.int64)[0])
<type 'numpy.int64'>