Uses a heuristic to determine if chord tremolos collide with accidentals.

Uses a heuristic to determine if chord tremolos collide with accidentals.

This heuristic makes several assumptions about when a chord tremolo
will collide with accidentals. It must be between whole notes, it
must be in the staff, and it must be an ascending major third or lower.
The heuristic is entirely contained in Beam::whole_note_close_chord_tremolo,
which should be modified if other cases involving chord tremolos arise.