add HeightMap class

Is this too specific to add to libgdx?
If it's good to add, I'm not sure collision is the best place.  Where should it go?
If it is merged, I will work on adding methods to ModelBuilder to create a 3d heightmap model

See it in action:
1. https://www.youtube.com/watch?v=xNbi63i8aT4
2. https://www.youtube.com/watch?v=1xwpW3-vW0Q