Merge pull request #426 from dstelter92/master

      // compute weights

      volume = domain->xprd * domain->yprd * domain->zprd;

      enth = pe + (pressref * volume);

      weight = log(set_lambda[my_set_lambda] + (eta*(enth + h0)));

      weight_cross = log(set_lambda[partner_set_lambda] + (eta*(enth + h0)));

      weight = log(set_lambda[my_set_lambda] + (eta*(enth - h0)));

      weight_cross = log(set_lambda[partner_set_lambda] + (eta*(enth - h0)));

      if (me_universe > partner) {

        MPI_Send(&weight,1,MPI_DOUBLE,partner,0,universe->uworld);

      }

      if (me_universe < partner) {

        boltz_factor = (weight - weight_partner + weight_cross - weight_cross_partner) *

        boltz_factor = (weight + weight_partner - weight_cross - weight_cross_partner) *

            (1 / (boltz * eta));

        if (boltz_factor >= 0.0) swap = 1;

        else if (ranboltz->uniform() < exp(boltz_factor)) swap = 1;