SYSML2_ — Symbol for metadata def is missing

Add support for defining a metadata definition using the normal definition symbol (e.g., key word metadata def with metadata def name in name compartment). Separate compartment for attribute declarations. Add compartment redefined the annotated elements so they are constrained to be of a certain metaclass. (Note that MetadataDefinition is a kind of ItemDefinition but MetadataFeature (and therefore MetadataUsage) is a kind of AnnotatingElement.)