Import and export of figured bass elements from MEI and musicxml file formats

[mxordn]

(1) Added the possibility to import musicxml and MEI representations of figured bass notations.

(2) Proposes also a class to represent Figured Bass in a music21 stream. The class is called FiguredBassIndication and inherits from the harmony object using the already existing objects from the figuredbass module.

Any comments are highly appreciated. A discussion in the google list is here available: https://groups.google.com/g/music21list/c/-Z_UQfZ9SPk/m/gyE9DciUCgAJ

[gregchapman-dev]

Quick comment/worry about mei/base.py: I think you started from an older version; you are missing a bunch of stuff that happened in the last year or so (for example the metadata changes I made in v8, and all the typing changes Myke made as well).

[mxordn]

Quick comment/worry about mei/base.py: I think you started from an older version; you are missing a bunch of stuff that happened in the last year or so (for example the metadata changes I made in v8, and all the typing changes Myke made as well).

This is interesting. I synced with the main repo some weeks ago and thought I used the mei/base.py that is in the current master branch. I will have a look at it.

UPDATE: I updated mei/base.py to the newer version. The newer parts didn't affect my changes.

[mscuthbert]

Hi. This PR currently changes 200+ files and has hundreds of commits. This won't be possible to review. Can you close and open a new one just with the changes from the "master" branch?

[mxordn]

As I came back to this topic today, I thought similar. Rebasing my fork to the new version triggered the checks here I think. I'll close this PR and open a new one when I have the stuff ready.