XTCE13 — Add gap entry to container entry list

A gap entry is an addressing mechanism to move the address pointer relative or absolutely as specified in the gap entry. This is an alternate approach to XTCE's location in container element. Reported by JPL.

Gap Entries For Sequence Container Not Needed

The original reporter for this issue has suggested that the driver for this is obsolete. Further discussion by the committee has not identified a need for this with the current syntax being completely able to capture gaps, albeit in a slightly different way. Propose to close this without change.