Tentative plan for thermodynamic consistency and for potentials as needed for kinetic phase transitions

This is a follow up and summary of a discussion had between myself, @chadmeyer @aematts @jhp-lanl . This is the path forward for potentials as needed for @aematts 's kinetic phase transition treatment:

• We will expose two new functions: EntropyFromDensityEnergy and EntropyFromDensityTemperature. Not all non-tabulated equations of state have an entropy in closed form. For now we will provide a stub in the base class that simply raises an error for those equations of state.
• We will provide an introspection mechanism so a downstream code can ask an EOS if it (a) has an entropy, (b) if that entropy is thermodynamically consistent, or (c) none of the above. A function with no inputs and an enum class output probably works.
• For tabulated EOS's we can pass through to the underlying tabulated information, e.g., through EOSPAC.
• In the medium term we will also look into providing a tool that can plot "distance from thermodynamic consistency" for a given EOS, perhaps as a python script, so users can decide for themselves if they want to trust a given EOS or table for a given problem/application. @aematts mentioned that PEM-EOS may already have such a tool they may be willing to share.
• In the longish term, we will consider the possibility of computing an entropy by integration and tabulating it, even for analytic EOS's with no closed form. This could perhaps be a modifier.

Thanks everyone for a productive discussion today. Please let me know if this summarizes the team's thoughts and whether or not it still makes sense.