Add command(s) to edit program settings

[alexdewar]

Edit: Moving the settings file out of the model directory is now a separate issue: #513

Currently the program settings are read from a settings.toml file in the model directory, if one exists (otherwise defaults are used). Right now, there's only the one (for setting the log level), but in future there will be more (e.g. setting the default output directory etc.).

The settings config file really shouldn't live in the model directory, which should be solely for model-related parameters, not settings for the whole program.¹ In particular, if users set things like file paths in this config file, then whoever tries to load the model on a different machine will run into issues: there will be an error if the path doesn't exist, but if it does, it may mean that the output files are stored somewhere unexpected, which is also not ideal.

It would be better to store this file in the program's config directory (e.g. ~/.config/muse2 on Linux -- you can use something like the dirs crate to choose an OS-appropriate one). The problem with doing this is that it may not be obvious to users where this file is -- especially less technically minded ones. So I suggest that we add a few simple commands to muse2 to help users with this, e.g.:

• muse2 settings edit - Open the settings file in the user's default editor, creating it from a default one if it doesn't exist
• muse2 settings path - Print the path to the settings file
• muse2 settings delete - Delete the file and in order to reset the program to factory settings

There are probably a bunch of these commands that we can imagine, but some of them may prove controversial and we don't want to overcomplicate things, so maybe we could just start off by adding the one to edit the settings file and seeing how we go from there.

Footnotes

1. This comes up in other projects (e.g. HealthGPS mixes program settings and model parameters in the same config file) and I hate it 🙃. If you open a Word document that someone's sent you, you don't expect it to start rearranging your toolbars or whatever, because your program settings shouldn't be something Word documents mess with. This is the same deal. /rant ↩

[alexdewar]

@Sahil590 I'm guessing you won't have time to work on this for this iteration if you're not working on MUSE any more?

[Sahil590]

@alexdewar I was going to finish this up if I had time, this sprint. Still working on #272. Does that work, or would like someone else to have a look at this one ? Or put in the backlog

[alexdewar]

Sure, that's fine! I'll move it to this iteration then.

For now, I think it's fine to load the settings.toml from the CWD (i.e. let's just put it in the repo's root). Longer term, we probably want it in a proper configuration dir (e.g. using the dirs crate).

I think it probably makes sense to work on the muse2 settings edit command first. You could do something like I did for clockify-tui to try to find an editor on the user's system: https://github.com/alexdewar/clockify-tui/blob/c78a9416fad807d88749092bf0c2138d267dfef1/clockify_tui/config.py#L123