Check length of compositions, output type

needed a force eval, () needed to be moved

Method from Ulrich Deiters, superancillary from our new paper

And JSON I/O

Also add the accessors as methods of the class in the python interface (overload resolution has large runtime overhead)

See #2

See #2

See #2

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

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