Removed useless argument, added tests for wigner.racah(), Fixed error for half integers

Added tests for ```racah()``` of ```sympy.physics.wigner``` module which was not added in https://github.com/sympy/sympy/pull/2878
Removed useless prec argument from clebsch_gordan and wigner_3j.
Updated docstrings.
Fixes https://github.com/sympy/sympy/issues/2767
Fixes https://github.com/sympy/sympy/issues/2766