Fix single point ts

[refszkentla]

fixes #1498; also fixes IndexError exceptions when an aggregation switch is absent in Config. (Config handles missing switch with its getattr() but if the switch is missing - e.g. in the case of no aggregation ordered - trying to access the switch list item raises IndexError)

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]

[refszkentla]

I did not find any particular tests for loaders, so did not change/add any tests.
This is not a new feature, so I have not filled any doc.
Once I have implemented the single aggregation IndexError fix, I asked AI model to populate across other aggregation loops in submodules in loaders/ folder ;). I do not know what is the intended way to attribute it.

[refszkentla]

one more thing. Actually, I am not quite sure why the single point recipe needs SUBAREA_ variables, i guess it could be safely removed. But, since I am not sure, i kept it.

[David Flack (daflack)]

Thanks for raising this PR. I'm hoping to get around to review it on Thursday. To answer your question around attributing AI, all we require is that if AI has been used it is mention in the PR. So I can confirm that you have attributed it in the intended way.