Adapted timing for openmp mulithreading.
@all I suggest to change the current timing which is based on calls to the subroutine cpu_time to a timing based on calls to the subroutine system_clock. At the moment, the measured time is in general not the elapsed wall clock time (which we want for accounting) but the (potentially accumulated) time during which the CPU did some work. This difference matters in case of openmp multithreading.