Set a timer.
        
        Args:
            num_secs(int): Number of seconds

def set_timer(self, num_secs):
        """
        Set a timer.
        
        Args:
            num_secs(int): Number of seconds
        """
        # FIXME / TODO support schemas? Accept timer id number as parameter?

        # Dumb heuristic; Query status, pick last device id as that is probably the timer
        status = self.status()
        devices = status['dps']
        devices_numbers = list(devices.keys())
        devices_numbers.sort()
        dps_id = devices_numbers[-1]

        payload = self.generate_payload(SET, {dps_id:num_secs})

        data = self._send_receive(payload)
        log.debug('set_timer received data=%r', data)
        return data