Add support for 'chord' keyword

[marksweiss]

• Would be a container for 1 or more Notes
• Would also guarantee that all Notes in the Chord have the same start time when the Chord is played
• Must have equivalent interface to Note everywhere, so that all operations that currently modify Notes after they are created word, e.g. code that iteraters over Notes to read/write properties on each one
• This suggests must derive from Note and override to make sure all operations are done on all Notes in the Chord
• This came up with MIDI import because notes as part of chord in imported score with not exactly same start time are not played together, but it extends very importantly to the fact that all scores that want to not set start times and let Players or Composer @cur_start automatically set start times have no mechanism for chords or polyphony of any kind -- they can't specify notes as starting together whatever that start time is