Set the group brightness.

        :param brightness: Brightness in decimal percent (0.0-1.0).

def brightness(self, brightness):
        """ Set the group brightness.

        :param brightness: Brightness in decimal percent (0.0-1.0).
        """
        if brightness < 0 or brightness > 1:
            raise ValueError("Brightness must be a percentage "
                             "represented as decimal 0-1.0")
        self._brightness = brightness
        cmd = self.command_set.brightness(brightness)
        self.send(cmd)