Shashlik out of time pileup mitigation

Multifit for ecal pulse shapes backport from 7_something, and PF pileup mitigation changes.  Includes #5845.

@bachtis, I had to add a change to class checksums
@lgray, @bendavid, FYI
@lgray, was this intended for HCal Phase1?  This function is only called by customise_HcalPhase1.  Currently it's causing the failure in the 2017 scenario (workflow 100xx).