ENH: interpolate: implement spline derivatives and antiderivatives

Implement routines for computing B-splines representing derivatives and
antiderivatives of B-splines.
ENH: interpolate: add derivative+antiderivative to UnivariateSpline, too
BUG: interpolate: refuse to differentiate order 0 splines (which are not differentiable)
STY: interpolate: get rid of repeated packing/unpacking in splder
BUG: interpolate: refuse in splder to differentiate splines with too many repeated knots
DOC: interpolate: fix doc issues in splder/splantider
TST: interpolate: fix splder/splantider tests
Merge pull request #2445 from pv/splder

ENH: interpolate: implement spline derivatives and antiderivatives