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

Also allow paths to be optional

Previously you always got Lorentz-Berthelot no matter what

Not yet fixed for mixtures

Fix reducing density for nitrogen; see https://github.com/CoolProp/CoolProp/commit/fe7f6db631f373d1c49948b0ca26b8592b3bbf15