Convert filter multiple cubes from extract_cube to extract

[daflack]

Switching to cube.extract() rather than cube.extract_cube() to enable multiple models to have derived diagnostics calculated.

Fixes #1791

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• 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.
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[github-actions]

[daflack]

Looks like it has worked, and appears to work for a recipe I used that was failing due to this error requiring input fields A and B to calculate something (or at least produced a new error that will need fixing so will raise an issue for that shortly). I'm not sure if I have done exactly what you were aiming at, and am not convinced with the tests update.