Make simulated rover holonomic

[daweim0]

Right now the simulated rover is a 4 wheel tank drive. The new rover this year is going to be swerve drive, so the simulator needs to be updated.

I also think we should not simulate the actual driving kinematics. Instead, we should make a floating box to represent the rover that perfectly executes commanded twists. Simulating the driving dynamics brings up a bunch of problems that don't exist in real life, so I don't think it's worth the effort.

This is a very important task. Contact David if you're interested.