closes #33

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

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

Add conversion routines for ideal-gas Helmholtz terms from CoolProp format

And add test

And to python interface

* Add partial molar volume as a possible output

* And expose to python

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

Not yet fixed for mixtures