Second bunch of miniAOD 70X changes post CSA14

This PR implements another bunch of items from
https://twiki.cern.ch/twiki/bin/view/CMS/MiniAODCSA14FollowUp


- Fix heavy flavour selection in pruned gen parts 
- Add photons first mother for pt > 10 (needed for pi0 decays)
- Add first mother for electrons with pt >3 (needed for pi0 decys)
- GenJet constituents rekeying (needed to enlarge the maxEta of packed candidates for fwd jets)

@gpetruc