- We need to use barrier after force/velocity calculations and
  just after the barrier start the parallel update

- The end and start values dependent on the thread ids, lead to
  race conditions if num of pedestrians is smaller then number of
  threads (pedestrians get deleted with forthcoming simulation)

- @todo: other models need the barrier mechanism too. not done in this
  commit

- @todo: Dispense with manually distributing the threads and use
  a "#pragma omp for"