Removed mono1.spanStartNow (unused variable)
Revised sequence.finshSpanSilently()

Score performances working correctly, but
still debugging Mono1.js.
Added the following functions:
track.prototype.currentMsPosition(),
track.prototype.advanceCurrentMoment(),
midiChord.prototype.advanceCurrentMoment(),
midiRest.prototype.advanceCurrentMoment().
Also revised some other parts of MidiChord.js
and MidiRest.js.