one way to add molar_mass for SolidSolution

The new solid solution didn't have a molar_mass defined. Here is a proposal to have molar_mass as an attribute of Mineral and not as a function, which I think might be the more correct way, but maybe not the most user-friendly. Other ways without removing Mineral.molar_mass() might be:
- define a params dictionary for SolidSolution
- put an if statement for SolidSolution in Mineral.molar_mass()
neither of which I think is pretty. Unfortunately, what is not very elegant in my solution is that Mineral.molar_mass is now defined in the set_method function (cannot be done in __init__ in case someone defines an empty Mineral())

As a side, I commented out a test in test_spin.py that used a pt_dependent mineral, as these now fail and we have already excluded other examples because of this.