Flop and Roll added

-Fixed height sensors
-Added secondary (plane+box) ground for height sensors
-Added flop and roll  method to applyActions

-Still missing learning for flop and roll.
Includes crater class for compilation
Includes additional crater model
Adding Crater class to escape from
Minor edits, including prevention of some illegal final positions of model
Formatting
Merge branch 'master' of github.com:NASA-Tensegrity-Robotics-Toolkit/NTRTsim into crater
Added surrounding crater for tensegrity to escape out of
Commenting
Minor tweaks
Commented out assertion temporarily
Got marker in the right position by referencing btRigidBody in model constructor. Will refactor this after ICRA
Applying force with abstractMarker
first attempt at caterpillar
new attempt to connect with rigids, will do muscles later
Ramp function for force
Cleaning up documentation
Merge branch 'master' of https://github.com/NASA-Tensegrity-Robotics-Toolkit/NTRTsim
Minor tweaks
Fixed the reset seg fault problem

On teardown, cleared the vector of objects that should be destroyed
with the reset