By adding the necessary Eigen header for real type of autodiff

Based largely upon: https://pubs.acs.org/doi/abs/10.1021/acs.iecr.1c04703

Next is the cross derivatives

Fix the parametric tracer; it was switching directions because the dot product check was not implemented properly.

TODO: add some tests to ensure this stays fixed, maybe P-R for CO2+ethane?