Add Run 2 average lumi, pileup, prefire; QCDscales for BSM signal

Add to systemtatics_master.yml: lumi_13TeV, CMS_pileup_13TeV, CMS_l1_ecal_prefiring to accommodate analyses in which data-taking years are not differentiated in at the combine level. Instead they are derived by running on the full Run2 sample with the conditions-dependent systematics filled event-by-event, to derive a sample-average value. Add option 'signal' to the QCDscale_ variables, to accommodate the targeted BSM processes in searches. Use case: SUS-20-004.