Add R(V) calc function to G16

[karllark]

G16 model has R(V)_A and f_A as inputs. From these, the R(V) of the G16 mixture model can be calculated.

Maybe also add in the inverse with a way to designated when a value of R(V) is not allowed. This happens for f_A < 1 as R(V)_B is fixed (f_A = 0 model).