DART Xtra interface docs

Here are the current calls into the DART Xtra.

The commands that deal with cameras, video and marker tracking within video. Currently, only one camera can be open at a time.

The commands that deal with the external devices and shared memory objects, provided by the VRPN system. There are a few commands that wrap up the basic part of the system.

Commands for accessing devices as a "client" (ie. for being notified when the device value changes). All access, even for devices attached to the local machine, use these commands. See the VRPN documentation if you are curious why.