iiasa
Introduce GAMS I/O for `IXMP4Backend`
This PR adds the next step of functionality for the new IXMP4Backend: I/O for GAMS. I can't yet confirm that the functions here cover all functionality, but locally, the scenario.solve() step in make_dantzig() works.
The PR also adds delete_item() to IXMP4Backend since that was still needed before getting to the GAMS I/O. The GAMSModel always want to record the package versions and remove the sets later on. As it turns out, we don't need to store the package versions in an ixmp4 DB, but I only noticed that after adding the functionality.
The whole make_dantzig() function still fails, however, because it's also trying to add timeseries data, which I will look into next. Handling timeseries data is more on a platform-level, I would say, that's why I want to do that in a separate PR.
The commits here are not very clean, either, sorry for that. I figure that's okay since no one else is working on these files at the moment.
How to review
TBD
PR checklist
- [ ] Continuous integration checks all ✅
- [ ] Add or expand tests; coverage checks both ✅
- [ ] Add, expand, or update documentation.
- [ ] Update release notes.
