The special function states should offer deriv2() for 2nd derivative #152
Description
All the classical diffeq states, cyl_bessel, sph_bessel, airy, etc could offer (say for cyl_bessel)
_Tp __J_deriv2() const;
_Tp __N_deriv2() const;
using the state variables.
Do this for orthogonal polynomials too.
I added deriv() to those already (mostly).