simplify procedure for making unique events

[aminnj]

When making MC, the seed is set automatically for each job randomly. But if the event number starts counting from 1 for each job, running on multiple files in a subsequent step may throw away events because it thinks they are duplicates. We get around this via

ProjectMetis/metis/CMSSWTask.py

Line 136 in 0011467

firstevt = 1 + (index - 1) * self.events_per_output

(i.e., first event in a job is forced to be non-overlapping). We can simplify this by removing the need for firstEvent to be passed into the job and just use something like

process.source.firstRun = cms.untracked.uint32(${IFILE})

since an event is unique if (run,lumi,event) is unique. $IFILE is already passed into the job (the job index), so we don't need an extra parameter for firstEvent.