Improve and fix the reporting in MDTest. (#336)

* Improve and fix the reporting in MDTest. Reporting per rank now outputs the performance of the individual rank (before the barrier), the iteration throughput includes the barrier time (if not -B=0 is set). Before, it was the time after the barrier.
* Clarify the computation of the results.
* MDTest improve CSV output to include aggregated result.

Author: JulianKunkel