Add shear to solidsolution

Not ready to merge, as the change is commented out at the moment, but I'll put this up for discussion.

As we've discussed, there is no thermodynamically required way to average G in a solid solution.  I'd probably prefer arithmetic averaging, but am willing to be convinced otherwise.  The reasons are (1) this is like the iso-strain (Voigt) bound, which might (?) be better for passing seismic waves, and (2), this appears to be what Hefesto uses (see SLB2011, fig 4).

Thoughts?