Aux coord aggregation fix

[mo-tomosevans]

Proposed solution to address #2023.

Contribution checklist

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

• Documentation has been updated to reflect change.
• New code has tests, and affected old tests have been updated.
• All tests and CI checks pass.
• Ensured the pull request title is descriptive.
• Ensure rose-suite.conf.example has been updated if new diagnostic added.
• Conda lock files have been updated if dependencies have changed.
• Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• Marked the PR as ready to review.

[mo-tomosevans]

The changes here apply the method used in the UK Trialling version of the RES toolbox for data which does not have time in a dimension coordinate but need aggregating.

I've created new functions to enable this, and have also included an argument for ensure_aggregatable_across_cases which is then able to proceed with aggregating cubes as long as they have time coordinates as either DimCoords or AuxCoords.
Happy to discuss a better option than including an argument in this function if there's a preferred method.

Running these changes generated aggregated plots for ukv and mogrepsuk data.

[github-actions]

[mo-tomosevans]

Need to add tests for changes in aggregate and _utils operator scripts to increase coverage