Add a reference to a memory data object.

        :param CodeReference ref:   The reference.
        :return:                    None

def add_ref(self, ref):
        """
        Add a reference to a memory data object.

        :param CodeReference ref:   The reference.
        :return:                    None
        """

        self.refs[ref.insn_addr].append(ref)
        self.data_addr_to_ref[ref.memory_data.addr].append(ref)