Add Q-Q plot functionality

[daflack]

Generating a q-q plot for model comparisons and simple scatter plots.

Fixes #497

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

[github-actions[bot]]

[daflack]

As with comment on #497 my activity here will pause for a month. If anyone would like to please feel free to pick up from where I have left off following the list in the issue. The functionality at the moment consists of a basic q-q plot with an optional one-to-one line.

[daflack]

Current status: works on CLI for both UM and LFRic data; does not work through GUI.

Links to #730