Function check_octaves 

pub fn check_octaves(
    notes: &NoteList,
    track: &PitchTrack,
) -> (NoteList, OctaveCheck)

Checks every note against the median of the voiced f0 frames inside it, and returns the notes with their octave flag set.

A note within OCTAVE_AGREEMENT of the median is fine. A note an octave above it, within OCTAVE_TOLERANCE, moves down 12 semitones when the median lies within OCTAVE_TOLERANCE of OCTAVE_FIX_CEILING or below, and a note of that lower key with a higher velocity holds it within COPY_SLACK at each end. Any other gap keeps the pitch and sets the flag. A note with fewer than OCTAVE_MIN_FRAMES voiced frames keeps its flag unset.