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

Test that all derivatives match with those of finite differentiation to numerical precision (they do!)

needed a force eval, () needed to be moved

Method from Ulrich Deiters, superancillary from our new paper

See #2

See #2

See #2

And Python wrapper and tests

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

Isochoric derivatives moved into their own namespace