PDME — Problem creating certain objects

Summary: There"s a problem creating PartDataRelationship, PartDataIterationRelationship,
PartStructureRelationship and PartStructureIterationRelationship objects.

Unfortunately neither PartData nor PartStructure inherit from CosGraphs::Node
in any way. This makes connecting a CosGraphs::Role-derived interface (like
PartStructureOfPrcl) pretty difficult .

I suggest to make both PartData and PartStructure inherit from
PdmFramework::ManagedEntity. This would also provide the ability
to add attributes and identifiers via the Attributable and Identifiable
interfaces.