This commit creates air_modes.rx_path, which is the common portion
of the flowgraph once the sample source and possible resampler are
created. It takes baseband I/Q and emits Mode-S packets into the
supplied message queue.
* Added Flightgear multiplayer output interface to uhd_modes.py. This allows flight with live traffic in fgfs.
* Quaternion library borrowed from PyPi and modified to generate angle/axis representation and construct rotation quat from lat/lon to ECEF.
* Miscellaneous enhancements and cleanup to fix timestamps, add aircraft type field (seems to be unused anyway), turnrate info
