Refactor staff positioning interface

[trevorbaca]

• [ ] Set all unpitched music to staff position 0 by default rather than to C4.
• [ ] Allow staff-position chord-specification. So that baca.staff_positions([0, (0, 2), 1, (1, 3)]) will work. Also baca.staff_position((0, 2, 4)).
• [ ] Integer not_yet_pitched=None keywords in every pitch- and staff-position-setting function; remove baca.not_yet_pitched().