Use heightmap driver for ground-following

Still could use some cleanup of the calls to getTerrainHeight that send more parameters than necessary
Update heightmap to automatically-generated grayscale image
Store terrain object as property on rover - also make wheels VWF objects
Have rover wheels follow the ground

Conflicts:
	source/rover.js
Add setHeading method and initialize rover heading to 0 degrees
Add callMethod to scenario.js
Update scenarios to use callMethod for setHeading instead of setProperty rotation

Conflicts:
	source/scenario/scenario2a.vwf.yaml
	source/scenario/scenario2b.vwf.yaml
	source/scenario/scenario2c.vwf.yaml
	source/scenario/scenario2d.vwf.yaml
	source/scenario/scenario2e.vwf.yaml
	source/scenario/scenario2f.vwf.yaml
	source/scenario/scenario3a.vwf.yaml
	source/scenario/scenario3b.vwf.yaml
	source/scenario/scenario3c.vwf.yaml
	source/scenario/scenario3d.vwf.yaml
	source/scenario/scenario3e.vwf.yaml
Make rover's placeOnTerrain function a public method so it can be called by grid
Use rover's placeOnTerrain function when adding it to grid squares
Remove an unnecessary pick from rover terrain following to speed it up
Add sceneNode property to rover so we don't need to find it over and over
Use this.sceneNode property in rover.js so we don't have find it over and over

Conflicts:
	source/rover.js

Conflicts:
	source/rover.js
Add new collision terrain that follows the visible one more closely on the plain
New nicer collision_terrain.dae
Fix my lousy merging of conflicts
Switch to color heightmap.png
Have rover follow the terrain when it turns
Remove areas were I was forcing the starting heading of the rover per scenario
Add Brett's cleverer way of constraining the heading to 0-359.999 degrees
Reference "this.sceneNode" instead of "scene" since global "scene" was removed