Adds a wrapper for RMSE calculated using the scores package

[David Flack (daflack)]

Fixes #2105

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.

[github-actions]

[David Flack (daflack)]

last two tests are currently repeats of the equivalent version in the difference. However, the difference plot uses .data and so whilst the data is flipped the coordinates are not flipped. Here we need both the data and the coordinates flipped otherwise we cannot collapse for timeseries or spatial maps. I need to test what happens if we don't flip the grids before putting it through scores. Worst case scenario we would need to remake the cube with correct data. Or would regridding the cube onto the other cube help with this?