Derivatives included in VRPN output

Derivatives of pose data are now automatically supplied as part of VRPN output.

For greater reliability and robustness, VRPN clients that are able to make use of velocity and acceleration data can use this information directly from Tracker's VRPN output, rather than having to calculate this on the client side.

This is useful, if, for example, you want to use Vicon data within a dead reckoning algorithm, or other prediction algorithm, to estimate pose at a time that may or may not coincide with a Vicon frame.