Adds the TransformRef if it is not already added.
        :param transform_ref:
        :return:

def add_child_ref(self, transform_ref):
        """
        Adds the TransformRef if it is not already added.
        :param transform_ref:
        :return:
        """
        if not isinstance(transform_ref, TransformRef):
            raise ValueError("transform_ref must be of type TransformRef")

        for child in self.children:
            if child is transform_ref:
                return

        transform_ref.parent = self
        self.children.append(transform_ref)