Add MPI_EVENT_MODE option to steer generation of total number of events
When specifying EVENTS= with MPI_EVENT_MODE=1 Sherpa will generate events in total, while in the default MPI_EVENT_MODE=0 Sherpa will generate number_of_cores* events in MPI mode.
(cherry-picking commit a670d0fb.)
This is basically a cherry-pick of !170 (merged) from master.
Edited by Frank Siegert