Jet energy scale systematic

[stiegerb]

Right now we apply the JES systematic on the SV-jet level, see:
https://github.com/stiegerb/TopMassSecVtx/blob/master/src/LxyTreeAnalysis.cc#L1344-L1353

We should do this on the event level instead, i.e. how often does the event have 4 jets with scale down/up (e/µ) or 2 jets with scale down/up (ee/µµ/eµ). I.e. as we do for the btag/met weights.