Add figured bass numbers to Humdrum

[WolfgangDrescher]

Would it be possible to add figured bass numbers when converting to Humdrum?

I'm not sure if figured bass is already properly implemented in music21. It seems that it's currently not documented in the user's guide and I found some comments about missing support in the developer reference: https://web.mit.edu/music21/doc/developerReference/musicxmlTest.html#figured-bass

[gregchapman-dev]

It would be possible (although it might require work in music21 as well; I've done that before for various features). It will not happen for a bit; there are a few missing converter21 features that are higher on my priority list (pedal marks, fingering, rehearsal marks, and D.S./D.C./etc come to mind).

Notes for when this is done:

1. Add figured bass to the Humdrum/MEI importers and to the Humdrum/MEI exporters.
2. This would probably be a good moment to add chord symbols as well.

[gregchapman-dev]

For the record: chord symbols are supported in the latest release (but not figured bass yet).

[gregchapman-dev]

Rehearsal marks are on the develop branch, being prepared for release, and pedal marks too (but pedal marks will be disabled until my music21 pedal marks PR is accepted).

[MarkGotham]

Figured bass would be great, but has proven to be rather a headache and is far from reliably converted in otherwise-standard formats, so budget time if going there!