Support user/vendor-defined data models

[cmschmidt]

The -02 draft added the ability to support multiple data models in SW M&A messages. Currently, the draft states that the data model is identified using an index from a proposed IANA table. There are currently no accommodations for data models that are not listed in that IANA table. It has been suggested that it could be useful for vendors and administrators to have some way of specifying the use of other data models to support proprietary extensions or local needs.