Add shear modulus modification to equations of state without a shear modulus

[bobmyhill]

Many equations of state in BurnMan are volumetric equations of state, and as such do not include parameterisations of the shear modulus.

There are several techniques by which shear moduli can be added to an existing equation of state. For example, see Hacker and Abers (2004; https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2003GC000614). One could also adapt the SLB equation of state for this purpose (although the calculation of the relative thermal energy would generally require an iterative step to obtain the thermal pressure).

We should implement a property modifier that can be used to (re)define shear modulus, and by extension Vp and Vs, as a function of pressure and temperature.