Specify how to encode unpitched notes

[adrianholovaty]

Just starting an issue to track this to-do: we need to specify how to encode unpitched notes. I think this will be relatively straightforward; just need to do the work.

Here are relevant links in the MusicXML docs:

https://w3c.github.io/musicxml/musicxml-reference/elements/unpitched/
https://w3c.github.io/musicxml/musicxml-reference/examples/unpitched-element/

[adrianholovaty]

See #320 for a broader concept, percussion mappings. For music that is completely unpitched percussion, I think we should use that approach, as it's cleaner and more elegant.

I suppose this particular issue, on the other hand, is about ad hoc unpitched notes that appear in a part that is otherwise pitched. Like in vocal music that has a temporary section of talking, where the talking is notated using indeterminate pitch.

We discussed this in a recent co-chairs meeting and agreed it would make sense to use the new staff position data type.