Add Trigger ZFElectrons to ZFEvent

Each ZFinderEvent now stores a set of ZFinderElectrons for the trigger
objects most closely matched to the reco electrons. In the future these
can be used to make cuts on trigger values just like we can currently
make cuts based on truth values.
Add cutting on ZF Trigger Electrons

ZDefinitions can now contain trigger cuts, like "tpt>20" or "teta<2.5".
Merge branch 'trigger_variables'