Enhance level filter operator to return all vertical levels

[jwarner8]

Fixes #726

Enables us to filter by cubes with no vertical coordinate, cubeslists that have variables with multiple vertical coordinates.

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• [x] Documentation has been updated to reflect change.
• [x] New code has tests, and affected old tests have been updated.
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[github-actions[bot]]

[jwarner8]

This has been tested with UM data, ensemble data (UM), LFRic and some ML data. Some ML data is not working in the collate step, but I think this is due to metadata in the cubes (forecast_period) switching from hours to day, possibly not netcdf compliant. Also made some changes to transect based on these changes

[jfrost-mo]

Tests are just failing due to the DT Center website being down. I've opened a PR to remove that as a dependency in #749, which you could review if you fancy.