refactoring and fail proofing

- removed scene_data_node.name
	
	reason being that it was redundant, since description did the same job,
but more efficiently, and doesn t muddle scene_nodes data n model data	

	everything now goes through scene_data_node.ref_object.decription

	meant quite some refactoring

- more robust node tree management

	can now handle failed get_data_sub_tree = null