Solves ground NULL pointer issue, demonstrated in AppNestedBoxTetrahedrons in dev/btietz
Documentation for NULL ground
Transition to an empty ground class and test with a cast
Clean up comments
Merge pull request #87 from NASA-Tensegrity-Robotics-Toolkit/NullGround

Merging in tgEmptyGround and associated files. Cleanup of comments in the terrain library generally. Closes #86