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

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