Real-Valued Spherical Harmonics

some useful spherical harmonics routines

Features:
[1] real-valued spherical harmonics
using real coefficients
[2] support flexible band intervals
(l=0:dl:L) including both full band
SH (dl=1) and even order only SH (dl=2)
[3] evaluate both function values
f(theta,phi) and the derivatives
of f w.r.t theta and phi. Can be used
for finding peaks of spherical functions
[4] implement the spherical harmonics
rotation algorithm of Ivanic and Ruedenberg