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

Also not clear if this is the best way to include it.  Probably need the std::variant approach here too.