Mapping sequence_id/EWT to art::EventNumber_t (32b)

[eflumerf]

There is a potential issue with the narrowing conversion from 48b Event Window Tag to 32b Event Number in art. We should review the agreed-upon convention for converting between event window tags and art event numbers to avoid rollover situations which could confuse later processing.

[normanajn]

Andrew will think about and design/report on scheme for doing mappting between EWT and Run/Subrun enum.

[rlcee]

One solution is to define the event number as the different between EWT and EWT at begin subrun