Add event skipper to the source.

Add the standard event skipper from cmssw to the HGCalSlinkFromRawSource.

A simple way to use it is via the firstEventToProcess option in test_raw2reco.py.

More complex selection can be done by adding e.g., the following line, to test_raw2reco.py directly:

process.source.eventsToProcess = cms.untracked.VEventRange('run:lumi:event_start-run:lumi:event_end')