Describe the bug
WDDTW and DDTW both error out due to improper usage of numpy's diff operation. Note this only affects the python implementations of these distances - the cython versions do not have this error.

To Reproduce
Run either WDDTW or DDTW with two numpy multidimensional time series arrays (i.e. two 3d arrays).

derivative_dtw_distance(np.array([[1,2,3],[4,5,6]]), np.array([[7,8,9],[10,11,12]]))

Expected behavior
No error.

Additional context
The diff usage (iirc) is incorrect for computing the derivative anyway. We should change this to compute the derivative properly.