New electron ID variable

[ajgilbert]

Appears the new cut-based electron ID has a new variable we don't save at the moment:

https://twiki.cern.ch/twiki/bin/viewauth/CMS/CutBasedElectronIdentificationRun2#Working_points_for_2016_data_for

dEtaInSeed => appears to be calculated as:

http://cmslxr.fnal.gov/source/RecoEgamma/ElectronIdentification/plugins/cuts/GsfEleDEtaInSeedCut.cc?v=CMSSW_8_0_21#0030

also same definition here:
https://twiki.cern.ch/twiki/bin/view/CMS/HEEPElectronIdentificationRun2#Selection_Cuts_HEEP_V5_1

If we want to add this should check it works ok on miniaod (should do), i.e. that:

ele->superCluster().isNonnull() && ele->superCluster()->seed().isNonnull()

are both non-null.