Formula mistake in ElasticProperties.py

[wenlibin02]

The correct one: $$15 G_v = (C_{11}+C_{22}+C_{33}) - (C_{12} + C_{23} + C_{31}) + 3(C_{44} + C_{55} + C_{66})$$
The wrong one: Gv = (4 * (Cij[0,0] + Cij[1,1] + Cij[2,2]) - 4 * (Cij[0,1] + Cij[1,2] + Cij[2,0]) + 3 * (Cij[3,3] + Cij[4,4] + Cij[5,5]))/15