Create Visualizer::BodyFollower class.

To cause the camera to follow objects around in a visualization. The class is a
generalization of the BodyWatcher class in TimsBox.cpp. I have replaced
BodyWatcher in TimsBox.cpp with this new class. I ran TimsBox before and after the change and the camera behavior looks the same to me. I've also used this BodyFollower in the rolling disc example, and it works well there too.

I'm not sure where exactly I should put the new code; let me know if I should relocate the additions.

See some related discussion in #74. @sherm1 said "Attaching the camera rigidly to moving bodies doesn't usually work well either because then the image bounces around too much." I didn't do anything to account for this.