closes #33

Can also specify them as JSON-formatted string, to make fitting a bit easier, for instance

(no implicit conversion of nlohmann::json to filesystem)

Closes https://github.com/usnistgov/teqp/issues/15

* Add ability to access a single departure term in a multi-fluid model

* Bump version to dev

* Add Chebyshev2D departure term type

* Split out reducing and mutants to their own headers

* Remove unused function

Add runtime test

Add test and expose in Python

Added a mockup of how one would hardcode models into the shared library (a la CoolProp), dynamic loads just need to be replaced with hardcoded strings, probably encoded as binary

Add tests for all binary pairs
Add conversion script for HFC mixture xi and zeta

Also allow paths to be optional