Close the communication link.

def close_link(self):
        """Close the communication link."""
        logger.info('Closing link')
        if (self.link is not None):
            self.commander.send_setpoint(0, 0, 0, 0)
        if (self.link is not None):
            self.link.close()
            self.link = None
        self._answer_patterns = {}
        self.disconnected.call(self.link_uri)