By adding the necessary Eigen header for real type of autodiff

Next is the cross derivatives

Added a mockup of how one would hardcode models into the shared library (a la CoolProp), dynamic loads just need to be replaced with hardcoded strings, probably encoded as binary

Couple of inlines are needed to avoid duplication of functions between compilation units

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

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

Small computational speed bonus

(but maybe the compiler elides all the copies anyhow)

Some more testing is needed, but looks promising so far