Introduction of polymorphic nodes in object trees

• Some searches in object trees may return multiple nodes (e.g. "var.A.x" and "var.B.x" for "var.x")
• Only leaves in object trees can be the target of summary arcs.
• Polymorphic nodes aren't interconnected with data dependencies.
• ClassGraph's map now split into kinds of vertex.
• ClassGraph's vertex now generic.
• Polymorphic member nodes cannot be leaves in object trees.

Missing until merge:

• Javadoc for new and updated methods.
• Code review.