Athena++ stretched grids support

[jzuhone]

PR Summary

This PR provides support for stretched grids in Athena++. I've restored the test_disk test that was causing issues before.

Closes (or probably closes) several issues, including #1585, #3389, #3619, and #4557, because it removes the buggy code that was previously handling grids with non-uniform cell spacing.

PR Checklist

• New features are documented, with docstrings and narrative docs
• Adds a test for any bugs fixed. Adds tests for new features.

[neutrinoceros]

This is awesome ! I would prefer the new nonuniform attribute be private, but apart from that, this looks good to me !

[jzuhone]

I have no idea why the doc builds are failing.

[neutrinoceros]

@jzuhone it's not related to your PR, I tried a build on another PR and got the same error

[matthewturk]

I love the simplification in code here. Nice work! It all looks good to me.

[neutrinoceros]

@jzuhone I have a patch to fix docs build at #4564

I think we can reasonably merge your PR first but if you're not in a hurry it would probably be safer to merge #4564, then rerun docs here.

[neutrinoceros]

I'll merge this in a minute. Just a note that we need to track down the various issues you linked here and close them as needed.

[neutrinoceros]

Coming back to this. Given the sheer number of issues that are closed by this patch, I now think it could be considered for backporting, despite its unusual size for a bugfix.
What do you think, @matthewturk and @jzuhone ?

[matthewturk]

I'd be OK with that, but since it's kind of more of an actual feature I'm not sure it shouldn't just be a minor release.