Fix SEGFAULT in ZTriggerEfficiencies.cc

Although e0 was being tested, e1 was then called, leading to segfaults
when e1 was a nullptr.
Add ZTriggerEfficiencies to ZFinder

Single electron triggers are now given scale factors in MC.

NOTE: If the trigger is required in the definition for _BOTH_ electrons,
the weight will be incorrectly applied twice!