Add some forceeval and type double where possible

Thought it would maybe help with compilation

But not clear why this changes anything...

The key point is to speed up compilation, and make it easier to develop, by using the packaged C++ interface more extensively

Fix missing inline

Added multipolar terms; doesn't compile on mac anymore

Appears to be working, additional testing required. Generalizing to handle G&V and G&T correlation integrals remains

No warnings from teqp code, all from externals

Very subtle problem with handling derivatives at zero density

Another bugfix release is needed

Closes #34

Closes #38

Closes #40

Helpful for debugging

Tests should pass again

Can do binary mixture tracing now!

To allow for providing the mole fractions, in order to allow for the use with mixtures.

Still can't trace isotherms with mixtures yet