add option to pass flags to ogs5_run()

when executing ogs5_run() with MPI for parallel runs. The command to the OS sets the flag --oversubscribe which is not necessary. (I remember that on your machine @phit0 it did not run with the flag. However, the flag should be removed. Instead there should be an option to pass additional flags beside the number of processors set with -np when using MPI. Maybe --oversubscribe should be set automatically if it does not run without.