squared '(1-x2)' term in fugacity coeff, consistent with f2pCO2.R and p2...

...fCO2.R and Weiss (1974)

When I previously fixed the calculation of the fugacity coefficient to follow the Weiss (1974) formulation, I did it correctly in the new routines f2pCO2.R and p2fCO2.R. However, in carb.R I forgot to sqaure the "(1-x2)" term.  That has now been corrected.