Move ProcessEventsFromSource from publicdb into SAPPHiRE

This consolidates the subclass of ProcessEvents for publicdb into SAPPHiRE.
The publicdb can then call this subclass from SAPPHiRE,
as implemented in HiSPARC/publicdb#60.

Making t_trigger part of the default columns should not break any scripts.
It is a new column at the end and defaults to -1 if not assigned anything else.

Add subclass combining the WithTriggerOffset and FromSource subclasses.

What is missing/caveats?
If non-standard trigger settings are used the function does not adapt to that.
If the data filter (HiSPARC/hisparcdaq#23) is active the trace may be malformed and not find a trigger, or the wrong one.