ENH: Added option to change the tracking frequency in NDICAPI

[atracsys-sbt]

Tracking frequency could now be changed similarly to the measurement volume i.e. by specifying the setting in the configuration file.
Example:

<Device
Id="TrackerDevice"
Type="PolarisTracker"
NetworkHostname="169.254.x.x"
NetworkPort="8765"
MeasurementVolumeNumber="1"
TrackingFrequencyNumber="2"
ToolReferenceFrame="Tracker" >

This sets the Extended Volume (MeasurementVolumeNumber=1) and a tracking frequency of 60Hz (TrackingFrequencyNumber=2) for a Vega Polaris tracker.
For TrackingFrequencyNumber,
0 = 1/3 of frame frequency =>20Hz
1 = 1/2 of frame frequency =>30Hz
2 = 1/1 of frame frequency =>60Hz

[Sunderlandkyl]

Can you add the TrackingFrequencyNumber description to the device documentation: https://github.com/PlusToolkit/PlusLib/blob/master/src/Documentation/UserManual/DeviceNDI.dox.

[atracsys-sbt]

Device manual updated