Rollingdisc example

See for 2a13a1a comments (copied below for convenience). This isn't ready to be merged; I'm just opening the pull request so I can ask questions.

One more comment is that I think I'm accessing states the wrong way when setting the camera's location/orientation. See https://github.com/chrisdembia/simbody/blob/rollingdisc/Simbody/examples/ExampleRollingDisc.cpp#L46; is there a better way to get the x and z position of the contact point?

"I tried to follow @sherm1's advice about how to set the initial speed of the
wheel. Currently there may be more damping than we want. Also, I am trying to
follow the disc with the camera. However, I cannot seem to call both
setCameraTransform() and pointCameraAt(); only one or the other can be
satisfied."

I'll need to do another check about energy conservation; does its motion look realistic?