Code maintainability - utils/__init__.py split (?)

[stan-dot]

At the moment the class in question is 1870 lines of code. Some of them do with await logic, some are syntactic sugar, some format outputs (ProgressBar), callbacks (CallbackRegistry), and control signals (SigIntHandler).

Expected Behavior

We get about a dozen files in /utils instead.

Current Behavior

Easier to locate where a change needs to happen and to recognize what changed.

Possible Solution

Just a new file for every logical chunk - likely bundling 2-3 connected classes at a time, or a set of functions all dealing with one functionality.

[callumforrester]

There has been a lot of previous philosophical discussion on whether lots of small files or a few large files are more maintainable in Python projects. I think it is one of these things that will never have much consensus. This particular repo goes for the "few large files" route so I think we should stick to that. One thing we could do is put the utils code inside __init__.py in a more obvious place i.e. utils/_utils.py and shim via __all__, but not sure that would be worth the effort.

[callumforrester]

Reviewers: @RobertSchaffer1

[callumforrester]

@prjemian suggests breaking up further in #1723