Flattop distribution: particles in tail have wrong cutoff

For a flattop distribution the particles in the tail are calculated as follows (https://gitlab.psi.ch/OPAL/src/blob/master/src/Distribution/Distribution.cpp#L2660) introduced in 9c51f84b:

      while (!allow) {
            t = gsl_ran_gaussian_tail(randGen_m, 0, sigmaTFall_m);
            if (t <= sigmaTRise_m * cutoffR_m[2]) {
                t = -t + sigmaTFall_m * cutoffR_m[2];
                allow = true;
            }
        }

For the cutoff sigmaTRise_m is used instead of sigmaTFall_m, it should read:

            if (t <= sigmaTFall_m * cutoffR_m[2]) {