'Overload common function len(). Returns number of elements in array.
Examples
>>> from sympy import MutableDenseNDimArray
>>> a = MutableDenseNDimArray.zeros(3, 3)
>>> a
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> len(a)
9'

def __len__(self):

    return self._loop_size
