The DF17/18 values are generally more trustworthy as they have full
CRC coverage. Errors in the CRC bits of a DF0/4/16/20 message can
result in the contained altitude being attributed to the wrong aircraft.
Move ModeA/C demodulator to demod_2000 (decoding stays in mode_ac.c)
Remove dependency on interactive.c in stats.c
faup1090 then doesn't need interactive.c at all.
Magnitude conversion now happens immediately when sample data is
received, so there is no risk of newly received data clobbering old
data under CPU overload.
Track NUCp when we compute positions.
Do speed checks when we have an updated position with the same or worse NUCp
before accepting the new position. Don't do speed checks on new postions if
they improve NUCp - assume that the new position is better.
Include NUCp in aircraft.json
Gather stats on reasons for rejecting CPR data due to range/speed check failures.
Expire old positions if we have had no updates for 60 seconds.
Closes#16, closes#17.
Switch signalLevel back to a power measurement, don't put SNR in there.
But make it a 0.0 - 1.0 double so we're not scaling everywhere.
Adjust for the amplitude offset when calculating power.
Adapt everything else to the new scheme.
