Pulse matched filtering places a boxcar filter upstream of the
averager and preamble detector. The filter length is equivalent
to the number of samples in one Mode S chip (0.5us). This
technique enhances operation when using sample rates > 4Msps.
When using --pmf, the threshold may need to be reduced a dB or
two from what would have been optimal without --pmf.
* rx_path.py now takes 'use_pmf', defaults to False
* modes_rx has new CLI option '-p' or '--pmf' to turn on PMF
* modes_gui has new checkbox on Setup tab
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.
