Get the base offset (the key we are using to index objects covering the given offset) of a specific offset.

        :param int addr:
        :return:
        :rtype:  int or None

def get_base_addr(self, addr):
        """
        Get the base offset (the key we are using to index objects covering the given offset) of a specific offset.

        :param int addr:
        :return:
        :rtype:  int or None
        """

        base_addr, container = self._get_container(addr)
        if container is None:
            return None
        else:
            return base_addr