v0.2.0 [write midi]

• Writing back to midi files is now supported! (haven't been tested in various cases) @lzqlzzq @Yikai-Liao
• Convertion between different time unit by Score(s, ttype='tick') or s.to('quarter') @Yikai-Liao
• Add property degree for KeySignature, which gives you a miditoolkit and pretty_midi style keysignature @lzqlzzq