TODO: apply polishing in another variable if constant composition polishing fails

So fast...

It's slow. Quite slow.

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

Fully works!

Also allow paths to be optional

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

Needs some testing yet, and to be made a first-class citizen

See https://en.cppreference.com/w/cpp/types/result_of

Switch critical tracing to use one of the molar concentrations as the fixed parameter when polishing

Expose more critical methods in pybind11 interface

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