53 x add mva electron

Adds package EgammaAnalysis/ElectronTools, which hosts the MVA electron ID and corresponding changes in RecoEgamma/EgammaTools.
The MVA electron ID is the EGamma recommendation for electron analysis already for a long time, but lived in a UserCode area until spring this year. The effort to port it to CMSSW and add it to the official release cycle has been interrupted by the GIT migration (package fell out of the repository again).

This pull request also integrates #568, #569 and #579.