Added Chord Progressions
You can create chord progressions now. It supports custom ones with Codable
protocol, that you can store and use later.
let progression = ChordProgression.i_ii_vi_iv
let cSharpHarmonicMinorTriadsProgression = progression.chords(
for: cSharpHarmonicMinor,
harmonicField: .triad,
inversion: 0)