Adds an entity to the entity layer
        @type entity: L{Centity}
        @param entity: the entity object

def add_entity(self,entity):
        """
        Adds an entity to the entity layer
        @type entity: L{Centity}
        @param entity: the entity object
        """
        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)