Add Muon Acceptance flag to ZFinder

The "use_muon_acceptance" flag, when set, will make sure that Zs are
only made from a pair of reco electrons that satisfy:
    * pt>30 and |eta|<2.1
    * pt>20 and |eta|<2.4
If it is set to false, then only pt>20 is required for both reco
electrons (as was the case before).

If no reco Z is created for the event, then the event is not used, so no
cuts are required for the trigger or truth Z.