Add hybrid fragment handling for GPS
The SBND and ICARUS GPS board reader provides a small status fragment every one second. When running in "on demand" "pull" mode, we need to have at least the latest fragment included in the event, even if the fragment was already included in a past event. At times when there is a lull in the trigger rate, there may be multiple status fragments available to include in the event. Any fragment older than ~ 2 minutes can be discarded.