Return the weighted average of array over the specified axis.
Parameters:  a : array_like
axis : int, optional
weights : array_like, optional
returned : bool, optional


Returns:  average, [sum_of_weights] : {array_type, double}

Raises:  ZeroDivisionError :
TypeError :

See also
Examples
>>> data = range(1,5)
>>> data
[1, 2, 3, 4]
>>> np.average(data)
2.5
>>> np.average(range(1,11), weights=range(10,0,1))
4.0