Tidied voiceLeading module.

This assists in porting over many of the voice-leading algorithms to the
OffsetTree-based Verticality class.
Updated metadata cache.
Added Verticality.bassTimespan property.
Implemented ChordReducer.removeShortTimespans().
Added VerticalitySequence to analysis.offsetTree.

This is analogous to voiceLeading.VerticalityNTuplet.