code: def add_entity(self,entity):
          
          if self.entity_layer is None:
              self.entity_layer = Centities(type=self.type)
              self.root.append(self.entity_layer.get_node())
          self.entity_layer.add_entity(entity)
docs: Adds an entity to the entity layer
          @type entity: L{Centity}
          @param entity: the entity object