XTCE12 — Provide way to categorize all elements with @name into one or more groups

Outside of the SpaceSystem and systemName, a general mechanism for categorizing @name elements into one or groups is needed. Reported by JPL.

The ability to create categories sufficiently exists

Creating categories for organizing named items in the XTCE document exists in the schema and has been enhanced slightly in XTCE 1.2 by adding names to more areas.

AncillaryData is already used by several programs to create arbitrary categories that are not needed for export to other tool chains. This is sufficient for this purpose.

A previous addition to the schema to create new elements for this purpose was not accepted by ballot.