Add test and expose in Python

Based largely upon: https://pubs.acs.org/doi/abs/10.1021/acs.iecr.1c04703

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