It works!

* Add methods for calculation of pure fluid critical points

* Pass the template arguments along

* Expose the values and Jacobian

To check whether you have gotten to the critical point

For isothermal VLE tracing

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?

I think g++ getting confused by copy elision on return of Eigen temporary?

Method from Ulrich Deiters, superancillary from our new paper

See #2

See #2