Added IlluminanceDisplay

This creates an default display for sensor_msgs/Illuminance.

I'm mostly happy with all of the options available from PointCloud2, so I chose to use point_cloud_common for this.  It's just a sensor reading, so a single point is sufficient.

The only downsides are that the Channel Name should be set to "illuminance" by default.  Is there a way to do that (or to even set and hide Channel Name)?

This will be similar, so once this is approved, I'll add FluidPressure, RelativeHumidty, and Temperature.