Digitizer: Make digi time relative to the start of the time slice.

This MR changes the digi time from a global timestamp to one that is relative to the start of the time slice. A time offset is now also applied in event-by-event mode to prevent negative timestamps in digis. These changes are in preparation for the updated 8 byte STS digi (MR !355 (merged)).

Also see discussion in issue #2131.