Integration of MVA/NoPU MET in central release

This PR contains the MVA and NoPU code the JetMET group wants to make available centrally, and eventually computed at the RECO and miniAOD level after discussion with the RECO/AT group.

This PR add the package RecoMET/METPUSubtraction, and add the pat configuration files needed for the MET uncertainty tool.
Several fixes and cleaning are also done on the patCaloMET configuration, the pileupjetid configuration, the pileupJetid training needed by the MVA and NoPU MET.

This value is "unique", only defined here and only used here (except that it matches the beginning of the determination of jet energy resolution which are for now stored in an histogram). 
Not hardcoding it would have of course more sense from a logical point of view, but the value is self consistent and does not have any other occurrence in other files.