Rename `music_notes` class to `MusicNote`

[mself]

• Rename music_notes class to MusicNote
• Rename files musicnotes.* to music_note.*
• Change static functions in music_note.h to static members of MusicNote class
• Update all callers
• docstring updates
• Rename noteValue() to semitoneValue()
• Move deprecated functions to the bottom of MusicNote class
• Add docs for MusicNote::noteOctave
• Add docstrings for members of MusicNote Make MusicNote::init() private