Update the measured light level in lux.

def update(self):
        """Update the measured light level in lux."""
        if not self._continuous_sampling \
                or self._light_level < 0 \
                or self._operation_mode != self._mode:
            self._reset()
            self._set_mode(self._operation_mode)
            self._wait_for_result()
        self._light_level = self._get_result()
        if not self._continuous_sampling:
            self._power_down()