Send a group write to the given address.

        The method does not check if the address exists and the write request
        is valid.

def group_write(self, addr, data, dptsize=0):
        """Send a group write to the given address.

        The method does not check if the address exists and the write request
        is valid.
        """
        cemi = CEMIMessage()
        cemi.init_group_write(addr, data, dptsize)

        with self._lock:
            self.send_tunnelling_request(cemi)
            # Workaround for lost KNX packets
            if self._write_delay: 
                time.sleep(self._write_delay)