ENH: interpolate: implement spline derivatives and antiderivatives

The derivatives and antiderivatives of B-splines are also B-splines, and the differentiation and antidifferentiation formulas are well-known. This PR adds routines for computing the derivative/antiderivative B-splines for a given B-spline.

Useful for e.g. finding maxima/minima of known 1-D functions, etc.