https://wiki.jmonkeyengine.org/jme3/advanced/physics.html#create-a-collisionshape
https://wiki.jmonkeyengine.org/jme3/beginner/hello_terrain.html#what-is-a-heightmap
Deprecated. Look at Design Question - Terrain instead.
https://wiki.jmonkeyengine.org/jme3/advanced/terrain.html