Switched to using complex_to_mag_squared vs. mag. Based on an observation that
the optimal slicer threshold is 0.5(V**2)t. Less CPU load, better performance.
Some configurations of threshold and --pmf result in slightly worse
performance. Most result in better performance.
Re-tab air_modes_slicer.cc
Re-tab air_modes_preamble.cc
Preamble subtracts average (LPF'ed) input from samples. This has the effect only
of improving the slicer threshold to exactly 0.5.