Change properties implementation.

This note from: Daffodil Parser: Explorations of the DFDL Standard

A more dynamic approach should be used, in which properties are treated as keys in a dynamic data structure
rather than as Scala members. This would decouple scoping rules from the code used to
access the values of properties, allowing the code to be more rapidly adapted to new versions
of the specification, or for experimental extensions to the specification. This change requires a
significant redesign of the schema parser.

Suggest: lists of the properties should be scraped off of the XML-Schema-for-DFDL-Annotations - a schema that IBM is publishing as part of their contribution to the DFDL standard. I.e., some XPath expressions that grab the right stuff from this.

Scala code should be generated off of this base so as to obtain the best of both worlds: strongly typed enums for property values, members for property access, but still allowing more rapid evolution.