Add pickle backed load cache to fetch_fcst

[jfrost-mo]

This does the loading of the data into cubes (with all associated callbacks) once in the fetch_fcst step, vastly speeding up the loading during recipe baking.

The read operator is updated to use this cache when it is provided a directory, and that directory contains a loadcache.pickle file. Otherwise loading will continue as normal.

TODO

• Unit testing
• Validation that it is actually faster (currently running)
• The callbacks that realise data need fixing to not do that.

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.