implement simple JEC in our validation code

[jpata]

1. L1 attempts to correct for PU: simulate the same event with & without PU
2. L2L3 attempts to correct gen to reco: match genjets to recojets, derive a correction factor C(pt, eta)

As we currently don't have information about which particles are from PU in the samples, we can't do L1.
But we could fairly easily implement L2L3, as we already match recojets to genjets.
We just need to extract a pT dependent correction 1/mean(response) as a functions of pT and eta of the reco jet, and apply it on the fly on top of our reco jets.

https://iopscience.iop.org/article/10.1088/1742-6596/404/1/012013/pdf
https://cms-opendata-workshop.github.io/workshop-lesson-jetmet/03-corrections/index.html

[jpata]

Is L1 actually done on top of PUPPI? Maybe it's not needed.
https://indico.cern.ch/event/783781/contributions/3386619/attachments/1832738/3001906/HEP2019_JECs_MD.pdf