Reconstruct trigger time in traces

Add functions to process_events make arrival times relative to the timestamp.

Functionality:
- [ ] Detect if the traces are from a 2 or 4 detector station, which determines the trigger conditions
- [ ] Find the moment of trigger in the traces of an event
- [ ] Then use this 'trigger offset' to correct the determined arrival times in each detector
- [ ] This makes them relative to the event timestamp, instead of the start of the traces

Still needed:
- [ ] The ADC values corresponding to the -30 and -70 mV thresholds.