Add thermodynamic potentials

Not ready to be merged. Opening for comments/suggestions.
If I understand Holland & Powell 2011, the B-M 3rd order, modified-tait and vinet EoS, can all be generalized as an expansion of the murnaghan with different expressions for K'' in terms of K' and K0. 
Should we generalize these EoSs? 
And can we expand this to the shear modulus, and see what is implicitly assumed for G''?
And can we generalize the different choices made for the correction of thermal pressure in the different papers? Is there any reason that we shouldn't be able to mix and match with different assumptions for the cold part?
I feel like I might be missing something...