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