VEP_UCD for relating to an high energy event

[loumir]

We added various suggestions for dealing with an event related quantity:

1. instr.event
   but event does not belong to the instrument , it is rather an observed 'phenomenon' , so obs.event would do

S obs.event Related to a signal messenger detection (photon or particle)

thanks for improving the definition if needed ..

Then from this term , we can set UCDs for all measured quantities and labels for an event.

event grade : as meta.code.class;obs.event. ( defined from a detector pixel island, for instance)
event identifier : as meta.id;obs.event
event type: as meta.code.class;obs.event (defined according to some decision in the analysis pipeline)