XTCE13 — lack of Union construct (MER + ASIST)

MER supports a Union construct because their abstract data types live past decomm. ASIST also supports the same idea – that an abstract data type onboard the spacecraft MAY live past decomm. Although it is possible to let multiple parameters overlap, in a sense allowing for a Union in XTCE. The issue arises that validating software cannot differentiate this with a bug in a container specification. A union tag or element of some sort is needed.