ENH: scipy.special: improving sph_harm function

Rewriting the function in Cython using different routing, thus increasing the speed and precision of function

Adding backward compatibility to sph_harm for double type m and n
DOC: special: update sph_harm docs
GEN: special: regenerate ufuncs
Merge pull request #3692 from branch 'pr-3692'

ENH: scipy.special: Improving sph_harm function

Reimplement sph_harm in Cython.