CPAlgorithms: allow NBTAG selector to take also a custom jet selection
Allow to pass a custom jet selection to the EventSelection
keyword that cuts on the number of b-jets in the event.
The allowed setups are now e.g.:
JET_N_BTAG >= 1 # use overall jet selection and btagger
JET_N_BTAG GN2v01:FixedCutBEff_85 >= 1 # use overall jet selection and custom btagger
JET_N_BTAG jvtORloose >= 1 # use overall tagger and custom selection
JET_N_BTAG jvtORloose GN2v01:FixedCutBEff_85 >= 1 # use custom tagger and selection