Provide a means to configure the sensor ID of a VRPN head tracking device

[seifertm]

When specifying a VRPN tracker for head tracking in the configuration file, Equalizer assumes the first sensor (sensor 0) of that VRPN device to be the head tracking sensor. However, this is not always the case. Since the sensor IDs of different VRPN devices are determined by the respective administrator, a developer using the Equalizer framework has no (easy) way to specify the head tracking sensor to be used.

Therefore, I propose the addition of a sensor ID argument in the observer section of the configuration file.