Add third density derivatives to sat_Z_accuracy.cpp (remarkably, even better than the first derivative)

Add tests for all binary pairs
Add conversion script for HFC mixture xi and zeta

Not yet fixed for mixtures

See https://pubs.acs.org/doi/abs/10.1021/acs.iecr.1c00847

Evidently not returning a value from a function that should causes some weird behaviors with (ierr != 0) not working properly