Ensures callbacks do not realise data

[David Flack (daflack)]

Fixes are required to um winds callback for the rotation, and to the cloud base altitude callback.

Fixes #1984

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[github-actions]

[David Flack (daflack)]

The cloud base altitude callback alters the function to use that given in #1985 by Scott Wales (@ScottWales) . Does not close #1985.

[Scott Wales (ScottWales)]

Thanks, can we have a test showing that cube.has_lazy_data() is true before and after running each callback?

[David Flack (daflack)]

Great idea on the tests for lazy data before and after each callback. I'm just wondering about the data and size (a lot of the test data is currently not lazy when loading directly with iris and no callbacks) so we may need to add new data in a cube for each callback that is lazy. I'll have a chat to James Frost (@jfrost-mo) about this and then start doing the tests.

[James Frost (jfrost-mo)]

Looks like a sensible change, and avoiding realizing data would be a great enhancement. I agree that Scott's test suggestion would be a good way to both test this change and to prevent regressions in the callbacks going forwards, so I would like to see it before this is merged.

[James Frost (jfrost-mo)]

Since we are now explicitly importing dask we need to add it as a new dependency.