Resolve "OrbitThreader doesn't track far enough when TRACKBACK=true"

Issue #586 (closed). Needs also to be applied to the master branch.

src/Algorithms/ParallelTTracker.cpp

@@ -247,7 +247,7 @@ void ParallelTTracker::execute() {
                       -rmin(2),
                       itsBunch_m->getT(),
                       (back_track? -minTimeStep: minTimeStep),
-                      stepSizes_m.getFinalZStop() + std::copysign(1.0, dt) * 2 * rmax(2),
+                      stepSizes_m.getFinalZStop() + std::copysign(1.0, itsBunch_m->getdT()) * 2 * rmax(2),
                       itsOpalBeamline_m);
 
     oth.execute();