phase2.1: subspine issue

[bel28kent]

This is to create a branch to fix the subspine issue from musicxml2hum conversion

[bel28kent]

• [ ] correct ties drawn between voices either by putting signifiers in separate spines and removing doubled voices, or with linked ties if cross-staff
• [ ] Merge only one pair of subspines on a record
• [ ] Merge from right to left. E.g., 1(a) 1(b) 2(a) 2(b) (2c) should merge 2(b) and 2(c), then 2(b) and 2(a), then 1(a) and 1(b)
• [ ] add cross-staff beaming
• [ ] swap subspine assignment where appropriate
• [ ] add flipper filter
• [ ] add stem directions, but only ad hoc to preserve edition
• [ ] remove unnecessary rest-height signifiers

[bel28kent]

pieces to return to:

• [ ] op11_no14
• clef change issue
• [ ] op11_no16
• clef change issue
• [ ] op11_no17
• clef change issue
• [ ] op11_no18
• issue #37
• [ ] op11_no19
• clef change issue
• [ ] op11_no20
• swap spines
• [ ] op11_no23
• clef change issue
• [ ] op15_no04
• clef change issue
• [ ] op16_no01

• 40

• [ ] op16_no03

• 41

• [ ] op17_no01

• 42

• [ ] op22_no03
• clef change issue