Reconstruct trigger time in traces

Add functions to process_events make arrival times relative to the timestamp.

Functionality:
- [x] Detect if the traces are from a 2 or 4 detector station, which determines the trigger conditions
- [x] 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~~
- [x]  Create a new column in the results table which contains the found trigger offsets, so the user can choose to compensate if (s)he chooses to.

Still needed:
- [x] The ADC values corresponding to the -30 and -70 mV thresholds.
- [x] Tests

@davidfokkema @NorbertvanVeen @NiekSchultheiss