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

Expose more critical methods in pybind11 interface

I think g++ getting confused by copy elision on return of Eigen temporary?

Method from Ulrich Deiters, superancillary from our new paper

And JSON I/O

See #2

See #2

See #2

Isochoric derivatives moved into their own namespace