Spherical harmonics

Improved implementation of spherical harmonics. Based on the additions to the
orthogonal polynomials, the "old" Ylm could handle some symbolic parameters
but only in "expanded", explicit form. This PR adds mainly real symbolic objects
for representing "Ylm". For the moment they are called "Ynm" and a pure addition
but I'd like to replace the old "Ylm" once this is finished.

Please see the docstring to get an idea of the new capabilities.