Harmonise variable names from UM Stash to LFRic variable and long_name

[ukmo-huw-lewis]

Adding variable name callback to make use of LFRic variable standard names across CSET

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.
• [x] New code has tests, and affected old tests have been updated.
• [x] All tests and CI checks pass.
• [x] Ensured the pull request title is descriptive.
• [ ] 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.

[github-actions[bot]]

[ukmo-huw-lewis]

Agree with James' further technical suggestions, and have committed those to branch. Have successfully rerun tests. Return to @daflack for review check/sign-off prior to merging.

[ukmo-huw-lewis]

New test added, and checked working as intended. Ready for review