Imported patch for making spherical harmonic symbolic.
Merge branch 'u/jkeitel/15017' of trac.sagemath.org:sage into public/ticket/spherical_harmonic-15017
Minor review tweaks for spherical harmonic functions.
trac #16614: Non existence of some small BIBD
trac #16614: Merged with 6.3.beta6
trac #16614: new doctest
Trac #15017: Symbolic spherical harmonic

This ticket is for making `spherical_harmonic` a symbolic function.

URL: http://trac.sagemath.org/15017
Reported by: eviatarbach
Ticket author(s): Eviatar Bach
Reviewer(s): Jan Keitel, Travis Scrimshaw
Trac #16614: Non existence of some small BIBD

From the handbook, page 73.

Nathann

URL: http://trac.sagemath.org/16614
Reported by: ncohen
Ticket author(s): Nathann Cohen
Reviewer(s): Vincent Delecroix