[natnet] add a NatNet3 to IVY bridge in Python

The new version of the Optitrack software (Motive 2.0) comes with NatNet
version 3. A Python module is provided with NatNet for
serialization/deserialization of the data streamed over the network.
This module is slightly modified here for a better handling of the
initialization and for stopping the threads.
The PPRZLINK message sent is REMOTE_GPS_LOCAL, so the transformation to
global coordinate system is left to the airborne part.

conf/control_panel_example.xml

@@ -47,6 +47,7 @@
     <program name="Attitude Visualizer" command="sw/tools/attitude_viz.py"/>
     <program name="App Server" command="sw/ground_segment/tmtc/app_server"/>
     <program name="NatNet" command="sw/ground_segment/misc/natnet2ivy"/>
+    <program name="NatNet3" command="sw/ground_segment/python/natnet3.x/natnet2ivy.py"/>
     <program name="Ivy2Nmea" command="sw/ground_segment/tmtc/ivy2nmea">
       <arg flag="--port" constant="/dev/ttyUSB1"/>
       <arg flag="--id" constant="1"/>