Adjust to iiasa/ixmp4#219

[glatterf42]

This PR contains the current state of adjusting to iiasa/ixmp4#219, which includes some changes and a big rewrite. It doesn't yet contain all changes that the shim layer requires and I'm running out of time, that's why the current state is not working properly. @meksor and/or @khaeru, I suspect it will be up to you to finish this.

Please note that I've imported various things as <their old name>. This is just to see how much behaviour changes in the refactor, please feel free to adjust these imports so that the shim layer uses the new names once we're satisfied.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• [ ] Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅
• [ ] Add, expand, or update documentation.
• [ ] Update release notes.