Allow trigger ID shift
We have discovered that there seems to be an offset of -1 between the TLU trigger ID and the trigger ID assigned to the M26 frame we are looking for. We discovered this when looking at very short CLICTD shutters where we saw time correlation (TLU trigger is at the correct time) but no spatial correlation (M26 frame gives us wrong pixels).
By shifting the trigger ID by -1, we basically select the M6 frame before - and have both spatial correlations (from the correct M26 frame) as well as time correlation (since the TLU timestamp is unchanged).
This MR allows you to shift the trigger IDs assigned to the event up or down.