Add osgUtil/Ground to autoplace a ground to a model with a fade out in the distance

The ground isn't prefectly set at the lowest height of the scene, so there's a slider to finetune it.
This is because I couldn't find a good way with the bounding box (not in same transformed space as the model)
I used the bounding sphere which works more consitently but with a small error.
WIP
Add ground blending with a background cubemap
Add comments and cleanup