Evaluate one derivative of the polynomial at the point x
x : array_like
Point or points at which to evaluate the derivatives
der : integer, optional
Which derivative to extract. This number includes the function value as 0th derivative.
d : ndarray
Derivative interpolated at the x-points. Shape of d is determined by replacing the interpolation axis in the original array with the shape of x.
This is computed by evaluating all derivatives up to the desired one (using self.derivatives()) and then discarding the rest.