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

And JSON I/O

And Python wrapper and tests

Scary warning from MSVC:
4>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.28.29333\include\cmath(638): warning C4717: 'copysign<double,double,0>': recursive on all control paths, function will cause runtime stack overflow

The functions for multi-fluid model need to be rewritten to build a temporary array, multiply by mole fractions, and sum.

Tracing is now crazy fast

Polishing missing still, otherwise done?

Doesn't work  so far