Chord duration correction

[ringw]

Currently, we detect notes with the same stem as a chord. We read the notes from left to right, and the duration of the chord is determined by the first notehead that we see. We should do something more robust, like take the median duration that we calculated for each of the component noteheads, and propagate it to all of the noteheads. We'll want to correct the glyph.note.end_time, glyph.type, and glyph.dots based on the chosen duration.