implement nEventsPerJob options for PrunDriver

In the past, when submit grid jobs by using PrunDriver, the available options are not allowed to split the jobs by number of events, now implemented, and already tested by submitting different kinds of grid jobs to validate it, see these 2 jobs:

https://bigpanda.cern.ch/task/34718078/ https://bigpanda.cern.ch/task/34651641/

The 34718078 task is the one using latest config (split the jobs by event number, 10000 per job, using job option 'nEventsPerJob'), and the 34651641 task is the one with job option 'nFilePerJob=1'. Their results are totally the same.