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
As expected, a build error is produce:
```
error: class 'susybsm::HSCPCaloInfo' has a different checksum for ClassVersion 12. Increment ClassVersion to 13 and assign it to checksum 1170741674
```
which of course means here to restore the old version.
I will reject this PR for the time being.