Unit test skips based on lack of optional module availability

[sadielbartholomew]

Add unittest skipping (including moving relevant imports and logic where necessary) in all cases where an optional dependencies (esmpy, scipy, matplotlib, access to the ncdump command via nco install or otherwise, etc.) are required, either as called in the test method itself or used under-the-hood in the cf logic.

This includes some fixes for cases where we already had such skips where esmpy is required, but there was logic at module-level which meant the unavailability of the module would be raised as an error before the test methods ran.

As tested locally via setting up a conda environment without any of the optionals in it:

$ conda create --name cf-313-nooptional python=3.13
$ conda activate cf-313-nooptional
$ conda install -c conda-forge cf-python udunits2

and ensuring we only have passes and skips.

(Thought I might as well sort this whilst reviewing our Python version minimum and maximum support.)

[davidhassell]

Looks good. scipy is not an optional dependency, so perhaps not put those changes in?

[sadielbartholomew]

Looks good. scipy is not an optional dependency, so perhaps not put those changes in?

Ah, good point, thanks! I seem to recall it used to be only optional, some years back (git blame on our requirements file seems to back this up) - hence why I thought it was. I'll revert any skips on SciPy.

[sadielbartholomew]

Hold the merge! Making some improvements in a few new commits...