Enhance functionality of IXMP4Backend

[glatterf42]

This PR expands the functionality of IXMP4Backend, bringing it closer to full parity with JDBCBackend. It does so by:

• adding implementations for some functions
• clarifying that some functions won't be supported
• migrating the ixmp4 tests to running on a PostgreSQL instance (enabling full functionality from ixmp4 in terms of versioning and rollback logic)

---

Setting up Postgres on GitHub Actions

The GHA runners only provide Postgres installations on Windows and Linux, but not on Macos. Thus, I'm trying to use this third-party action, which works on all OSs and is actively maintained.

How to review

• Read the #NOTE, #TODO and #FIXME marks in the diff and respond if necessary.
• Note that the CI checks all pass.
• Check the pytest marks @pytest.mark.ixmp4, @pytest.mark.ixmp4_not_yet, and @pytest.mark.ixmp4_never to see if the usage/explanations are clear.
• Read the documentation of differences between ixmp4 and JDBC and see if it's clear or if anything might be missing.
• Respond to comments I made on the files here on GitHub.

TBD:

• Once Document how to setup a local Postgres DB message_ix#981 is merged and we can link to the new docs section: Try setting up a local ixmp4 postgres DB and running some tests on that

PR checklist

• Continuous integration checks all ✅
• Add or expand tests ; coverage checks both ✅ (see below)
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

❌ Patch coverage is 96.81529% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.2%. Comparing base (bae4917) to head (95f8d72).
⚠️ Report is 49 commits behind head on main.

⚠️ Current head 95f8d72 differs from pull request most recent head 00d2867

Please upload reports for the commit 00d2867 to get more accurate results.

Files with missing lines	Patch %	Lines
ixmp/testing/__init__.py	85.9%	9 Missing ⚠️
ixmp/backend/ixmp4.py	97.0%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #601     +/-   ##
=======================================
- Coverage   98.5%   98.2%   -0.3%     
=======================================
  Files         51      51             
  Lines       6345    6557    +212     
=======================================
+ Hits        6251    6444    +193     
- Misses        94     113     +19     

Files with missing lines	Coverage Δ
ixmp/_config.py	94.5% <ø> (ø)
ixmp/backend/base.py	99.5% <100.0%> (+<0.1%)	⬆️
ixmp/backend/ixmp4_io.py	97.1% <100.0%> (+<0.1%)	⬆️
ixmp/backend/jdbc.py	97.3% <100.0%> (-0.1%)	⬇️
ixmp/core/scenario.py	99.2% <100.0%> (-0.4%)	⬇️
ixmp/core/timeseries.py	97.4% <100.0%> (+<0.1%)	⬆️
ixmp/tests/backend/test_base.py	98.0% <100.0%> (ø)
ixmp/tests/backend/test_ixmp4.py	100.0% <100.0%> (ø)
ixmp/tests/backend/test_ixmp4_io.py	100.0% <100.0%> (ø)
ixmp/tests/backend/test_jdbc.py	100.0% <100.0%> (ø)
... and 12 more

... and 1 file with indirect coverage changes

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[khaeru]

Here is the run in which all checks pass.

[glatterf42]

Thanks for picking this up and pushing it over the finish line :)

[khaeru]

• I note that it is no longer possible to run pytest at all without exactly the right local PostgreSQL installation. In a future PR, I'd:

  • document this requirement, and
  • try to relax this requirement, letting tests simply fail if the required database service is not available.

It turns out I/we should have done this before merging, as today workflows in message-ix, message-ix-models, and message_data are failing with the updated main branch.

I think this happens because ixmp.testing is used as a pytest plugin in those test suites, so ixmp.testing.pytest_sessionstart is run.

I'll open a PR to fix.