Add test and expose in Python

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

Not yet fixed for mixtures

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

Some more testing is needed, but looks promising so far

Test that all derivatives match with those of finite differentiation to numerical precision (they do!)

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