Peng-Robinson + Wong-Sandler (Gᴱ mixing) property package

The one Gᴱ rule that also reproduces the theoretically correct quadratic composition dependence of the second virial coefficient, so it stays sound at low density as well as high. Carries a kij alongside the activity model; like Huron-Vidal it performs best with parameters refit for it rather than the bundled low-pressure binaries.