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.

Python compilation working again

Break up pybind11 interface into separate compilation units (not as beneficial as hoped); still very slow