Ported flashgg to 7_2_x

Changes for 7_2_x: …
- switched to cutbased PUJetID algo due to TMVA XML compatibility issues
- updated ElectronProducer to new GsfTrack data format 
( @bendavid or @matteosan1 please check that I did the right thing here).  
- updated usage of ClusterTools
- updated instructions

I also had to disable the energy regression because of this issue.
https://hypernews.cern.ch/HyperNews/CMS/get/sw-develtools/2157/1/3/1.html
(the proposed workaround does not work with crab).