Fast Clebsch-Gordan coefficient function with limited accuracy. Use clebsch_gordan.m for machine-precision results. Inputs:
indices - CG coefficient indices, ordered as follows: [L M L1 M1 L2 M2]
Log-factorials are used to avoid numerical precision issues; the function works reliably to about L=100. If physically inadmissible indices are supplied, a zero is returned.