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

To be sure

Ready for Python!

Using sigma in A seems to be necessary to avoid underflow in some numerical calculations, as is done in PC-SAFT

Seems to be fixed now, not sure what the fix was

Add redirection include

Still need the model and so on, but working

n[21] was -0.99218575e+1 instead of -0.99218575e-1

This is why you should make sure it compiles...

clang on mac doesn't mind

Also test dipolar with JSON

The references seem to be getting broken. There must be an implicit copy somehow.