Refactor difference code cube handling

### Something else

In the difference code there is a fair amount of checking of the cube data. This process is repeated within the SSIM (#1768) and q-q plots (#1702) for model comparisons.

For ease, and in case others need to use it, it is suggested that the cube handling/checking is converted into a different function and put as a utility function.

Locate the cube-data checking in the difference code, then compare the repeated handling in the SSIM work from #1768 and q-q plots from #1702. Determine the shared behavior to extract into a utility, and confirm that the three model-comparison paths use it without changing their cube validation results.

Written by the indexing model from the issue text.