XTCE12 — 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.

Good idea but not needed in current scope

This is an interesting idea. The current method has shown to be implementable by several vendors, so there is not a defect. Adding this new requirement would impact vendor support/adoption time/effort for XTCE 1.2. Because this does not actually represent a defect, I propose to defer it to a "2.0" for re-consideration.