Add reference object in references under rtype/label=target

def add_reference(self, reftype: str, label: str, target):
        """ Add reference object in references under rtype/label=target """

        # The self.data[reftype] dict springs into being during the
        # register_references event handler at startup, which looks in the
        # kb registry for all registered reference names.
        self.data[reftype][label] = target