Closing in with the models

Hours later...

Yet another battle with typing

* A fully encapsulated library wrapper with C++ interface

* Refactor types.hpp to hide the big headers in a macro conditional section

By adding the necessary Eigen header for real type of autodiff

Next is the cross derivatives

Couple of inlines are needed to avoid duplication of functions between compilation units

Check length of compositions, output type

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

Scary warning from MSVC:
4>C:\Program Files (x86)\Microsoft Visual Studio\2019\Professional\VC\Tools\MSVC\14.28.29333\include\cmath(638): warning C4717: 'copysign<double,double,0>': recursive on all control paths, function will cause runtime stack overflow

Maybe a bug in Eigen that it can't figure out the type promotion