Online/Offline merge safety

If one has a merged (created by JMergeEvt) MC file, then the id is not unique and cannot mapped directly by the trigger_counter. Idea: if more than one id for a certain trigger_counter value is available, those events are matched by the timestamp. If this still yields more than one solution and exception is thrown.

added usability label

requested review from @tgal

assigned to @jschumann