Not sure it helps...

Closing in with the models

Attach model specific methods at model construction

Dropped three more esoteric ones with good replacements

Maybe add another smarter runtime method called model that will go through the model types and return the one that is there

Closing in

Also a method to give more helpful error methods for EOS-specific methods

Gets very close to what is needed for property Jacobian

Everything seems pretty promising aside from the mutant, which requires additional thought. How can we store the mutant in AllowedModels

Very fast in C++ (less than a microsecond for 4 outputs in the Jacobian), but 10 microseconds in Python for 2. Need to find a way to make it closer to original speed. Perhaps pass in a buffer to be populated.  That should allow the copy to be avoided. It must be the Eigen arrays that are so slow.