Not yet fixed for mixtures

See https://pubs.acs.org/doi/abs/10.1021/acs.iecr.1c00847

Evidently not returning a value from a function that should causes some weird behaviors with (ierr != 0) not working properly

Don't compile in clang

Some more testing is needed, but looks promising so far

Test that all derivatives match with those of finite differentiation to numerical precision (they do!)