adding ZmumuJets_Pt_20_300_GEN_13TeV_cfg.py to support 13 TeV relval set

Greetings Vitaliano and Josh, can you please consider accepting ZmumuJets_Pt_20_300_GEN_13TeV_cfg.py among the gen cards ? That sample is part of the relval set - most gen cards @13 TeV were imported in august, but this one was left out. Thanks and greetings. g.