Add mask operators

[daflack]

Adds a generate and apply mask operator.

Fixes #221

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.
• 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]

[daflack]

Three example recipes provided, workflow changes are not part of this pull request and may link to the need to add custom recipes to the workflow.

[jwarner8]

Just started reviewing this, looks like a great addition. Thanks for developing this. Think I am a bit unclear about apply_mask and how it interacts with multiple cubes/masks. Once I understand will complete review

[daflack]

Review comments completed @jfrost-mo and @jwarner8 back to you.

[jfrost-mo]

Overall looking like a good change. I've left some code comments, but nothing there is particularly fundimental.

I haven't looked at the tests yet; will do that later.

[daflack]

Converting to draft to remove all CubeList functionality as there is starting to be operator purpose creep.

[daflack]

Ready for review again following changes to functionality to only be applicable for cubes due to risk of operator creep. Review comments also taken into account @jfrost-mo and @jwarner8.