Issue #1928 add utvd support

[JoerivanEngelen]

Fixes #1928

Description

This adds support for the UTVD advection scheme for which @Manangka (and others) put in a lot of effort. I expect this advection scheme to be the scheme of choice in many cases as the example results here look very promising.. I didn't add another example, but modified an existing example of an unstructured tranport to use this scheme as it should be the advection scheme of choice for most of these models.

Checklist

• Links to correct issue
• Update changelog, if changes affect users
• PR title starts with Issue #nr, e.g. Issue #737
• Unit tests were added
• If feature added: Added/extended example
• If feature added: Added feature to API documentation
• If pixi.lock was changed: Ran pixi run generate-sbom and committed changes

[Copilot]

🟡 Changes recommended

Add the missing package-fixture and transport-splitting coverage, and update the stale module overview.

1 open finding

• Update module overview to document the fourth UTVD scheme · New

What changed in this PR

Adds public MODFLOW 6 UTVD advection support for unstructured transport models.

Changes:

• Adds and exports AdvectionUTVD.
• Updates the circle transport example.
• Adds tests, API documentation, and a changelog entry.

File	Reviewed changes
imod/​tests/​test_mf6/​test_mf6_adv.py	Adds UTVD rendering coverage.
imod/​mf6/​adv.py	Defines UTVD; module documentation and broader package/transport test coverage need updates.
imod/​mf6/​__init__.py	Exports AdvectionUTVD.
examples/​mf6/​circle_transport.py	Uses UTVD for unstructured transport.
docs/​api/​mf6.rst	Documents the new API.
docs/​api/​changelog.rst	Records the feature.

🧠 Review effort: Lite

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[LuukBlom]

LGTM!

Seems like no logic was added and this is pure doc updates + kernel support?

Make sure the examples on Teamcity are building correctly before merging. Not sure about how its setup so approving already.
Perhaps the latest kernel updates need to be in Teacmity or something?