XTCE12 — Add ArgumentTypeSet to MetaCommand

Allow for local argument types to be defined in MetaCommand, helps avoid collisions with arguments in other commands of the same name but different properties.

Defer to future XTCE revision

This is an interesting idea, but it seems to not be worthwhile for the extra effort needed to make this happen by vendor implementations. The current method does work as-is. Argument types do not need to be named the same as the Arguments, so name collision is avoidable, even with sharing of base types.

Perhaps this could be addressed in a model-based XTCE 2.0