Oddly this fixes the error in derivative, but it should have no impact

Temporaries were being returned when autodiff was being used, forceeval should fix it I think

It works!

* Add partial molar volume as a possible output

* And expose to python

To avoid unused variable warnings

By adding the necessary Eigen header for real type of autodiff

Next is the cross derivatives

Big speedup for gradient calculations

Method from Ulrich Deiters, superancillary from our new paper

And Python wrapper and tests

Add test for neff; it is undefined for vdW EOS, interesting