fraction. Now adds the two fractions.
This created a bug with rivers when mixing with the same solution. The mass fraction
was calculated incorrectly because only the second part of the solution was used (solution, frac 2).
git-svn-id: svn://136.177.114.72/svn_GW/phreeqcpp/trunk@2888 1feff8c3-07ed-0310-ac33-dd36852eb9cd