Part-combine: Add a way to override the part-combination decision

Part-combine: Add a way to override the part-combination decision

Add functions partcombineApart[Once], partcombineChords[Once],
partcombineUnisono[Once], partcombineAutomatic[Once] etc. to tweak the
decision of the part-combiner.
Internally, they are implemented using PartCombineForceEvent

These events are not processed by an engraver as most other events,
but rather directly in Scheme by the part-combiner.