Return string representation of the current state of the sensor.

def current_state_str(self):
        """Return string representation of the current state of the sensor."""
        if self.sample_ok:
            msg = ''
            temperature = self._get_value_opc_attr('temperature')
            if temperature is not None:
                msg += 'Temp: %s ºC, ' % temperature
            humidity = self._get_value_opc_attr('humidity')
            if humidity is not None:
                msg += 'Humid: %s %%, ' % humidity
            pressure = self._get_value_opc_attr('pressure')
            if pressure is not None:
                msg += 'Press: %s mb, ' % pressure
            light_level = self._get_value_opc_attr('light_level')
            if light_level is not None:
                msg += 'Light: %s lux, ' % light_level
            return msg[:-2]
        else:
            return "Bad sample"