Added DDiff function which extends numpy.diff function to discrete derivatives at any order and any accuracy

def DDiff(tab,h,DiffOrder,Accuracy):
    """
    DDiff(tab,h,DiffOrder,Accuracy) returns the discrete derivative of Order DiffOrder of the 1D array tab at the given Accuracy assuming a Finite Difference homogeneous h step. The size on the returned array is the same as that of the input one.
    It is en enhancement of the numpy.diff function.
    """