Adding correct Single Electron triggers, redux

[trtomei]

We follow the recommendations from the EGamma POG on Single Electron triggers:
https://twiki.cern.ch/twiki/bin/view/CMS/EgHLTRunIISummary

(doing it correctly this time)

[mcremone]

The list of triggers that was put together originally is home-cooked. For example, single electron triggers take into account the work done by the monojet group to recover some of the inefficiencies (reason why, for example, we have HLT_ECALHT800 in the list).

I like the idea of using single photon trigger to recover efficiency at high electron pt.

Bottom line, I would leave the lists as they were but adding single photon to the single electron paths.

[areinsvo]

This PR has been open for a long time. Can we revisit this and get it merged? At minimum we need to add the single photon trigger to 2017 and 2018 singleele_trigger_paths. If we are taking the trigger efficiencies from monojet, then we should copy their trigger list: https://github.com/bu-cms/bucoffea/blob/master/bucoffea/config/monojet.yaml