Implemented `analysis.offsetTree.Parentage.mergeWith(timespan)`.

The method generates a new parentage timespan from two similar contiguous
timespans.
Added `Parentage.part` property.

Sometimes two parts have the same name, so being able to compare parentages
based on the ID of the part is more useful.
Implemented `Parentage.pitches` property.
Improved OffsetTree's Verticality iterators.
Continued integrating OffsetTree into ChordReducer.