Make object_diagram:Model a concept (not an aspect)

examples/oml/SystemDesignOntology2Layers/src/oml/ua.be/sdo2l/vocabulary/formalisms/object_diagram.oml

@@ -25,7 +25,7 @@ vocabulary <http://ua.be/sdo2l/vocabulary/formalisms/object_diagram#> as object_
 		to element
 	]
 		
-	aspect Model []
+	concept Model []
 	
 	relation entity HasObject [
 		from Model