Support for --nbmake-by-cell

[blais]

Simple tests added, all tests pass.

(Note: I could simplify the format even more to require every cell in a unique test (instead of using section headings) if you like, but then what is an optional initialization cell isn't as clear - it would have to be marked somehow. Plus this forces test writers to insert a heading with a meaningful name.)

[blais]

Great comments, will resolve all. I'll check out isort and pyright, not familiar with those.

[blais]

Sorry for the delay, I've done without the library for now - too busy with the other things the project requires - contemplating whether to cook something custom instead of using the open source project. I may come back with the fixes and push this through.

[blais]

Hi! We ended up doing something else, and I think it never really caught on. (People got used to writing unit tests in regular Python files instead of notebooks. /shrug) I still think it's a powerful idea, but I have no direct use for it today, so I'll cancel this now.