Nemobug2 - bug fixes for a certain set of NEMO output - #138

Have just resolved a trivial conflict (in the Changelog) to help me to review from a clean state. Now closing and re-opening the PR to trigger the new CI jobs which will help me to see the influence on the tests across Python versions and platforms.

Going to build on top of the initial PR @bnlawrence, since for setting up the reference conda environment and the CI jobs to run the tests with and without image comparison, I have created a new directory of reference images specific to those expected to the reference environment, namely tests/v350-controlled-env-ref-images.

So I will remove the updated tests/reference-example-images plots here and copy any changes due to the underlying bug fix across to tests/v350-controlled-env-ref-images. And generally work towards getting this ready to merge ASAP. I'll address any review comments directly as new commits in the view to get this merged quickly.

Mid-way through updating the PR to account for the new reference environment and matched reference plots directory. Note the 'files change' count looks quite bloated because I am updating the images the docs point to here also since I'm adding a new example for the NEMO grid as part of work related to NEMO and the bug manifesting with it before this fix.