Mark subarea bounds as compulsory, so they show up when toggled

[jfrost-mo]

Also tweak some help text for CSET_CYCLE_PERIOD.

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.
• [x] All tests and CI checks pass.
• [x] Added an entry to the top of docs/source/changelog.rst
• [ ] Conda lock files have been updated if dependencies changed.
• [x] Marked the PR as ready to review.

[github-actions[bot]]

[jfrost-mo]

Actually there are more issues here: Opened #613 for this.

KeyError: ('Variable without a value.', '$SUBAREA_LAT_BOUND_BOTTOM')

[jfrost-mo]

Thanks for the review! I'll go ahead and merge this.