Extend UM vertical plotting to model level

[Sylviabohnenstengel]

Fixes https://github.com/MetOffice/CSET/issues/658 Um based vertical plotting on model levels UM based spatial contour plots on model levels

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [x] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [ ] All tests and CI checks pass.
• [x] Added an entry to the top of docs/source/changelog.rst
• [ ] Conda lock files have been updated if dependencies have changed.
• [ ] Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• [x] Marked the PR as ready to review.

[Sylviabohnenstengel]

ensure to review https://github.com/MetOffice/CSET/pull/638 first

[github-actions[bot]]

[Sylviabohnenstengel]

coverage report failing. This should be solved by addressing the coverage report issue in PR #638

[Sylviabohnenstengel]

ensure to review https://github.com/MetOffice/CSET/pull/638 first

[jfrost-mo]

Branch rebased.

[Sylviabohnenstengel]

corrected recipe to reflect new vertical level constraint