Peng-Robinson + Huron-Vidal (Gᴱ mixing) property package

The original infinite-pressure Gᴱ mixing rule (Huron & Vidal 1979) on Peng-Robinson. Rigorously it wants activity parameters regressed AT infinite pressure; driven by the bundled low-pressure NRTL binaries it is markedly better than a plain kij but measurably worse than MHV1 — prefer pr-mhv1 unless you are supplying your own infinite-pressure-fitted parameters.