Return weights for an Np-point central derivative.
Assumes equally-spaced function points.
If weights are in the vector w, then derivative is w * f(x-ho*dx) + ... + w[-1] * f(x+h0*dx)
Np : int
Number of points for the central derivative.
ndiv : int, optional
Number of divisions. Default is 1.
Can be inaccurate for large number of points.