Override del_construct to update cyclic axes set when due

[sadielbartholomew]

Fix #758 and #756 (the former being the underlying issue and the latter a specific bug as a downstream effect) by overriding the del_construct inherited from cfdm to account for and update the store of cyclic axes. Previously we used the inherited method as-is so cyclic axes weren't updated and could get out of sync, notably contain axes which no longer existed due to deletion.

Note:

• The test test_Domain_del_construct added here will currently fail halfway-through due to cyclic: different sets for Field.domain & corresponding Field #762, as noted in the test. I am fixing that next.
• I will add a note of two to the code in anticipation of some comments in review, for discussion.

[sadielbartholomew]

Just going to add one commit to fix a few style/formatting issues detected by the CI linting.

[sadielbartholomew]

@davidhassell one note to consider for your review. Thanks.

[davidhassell]

Hi Sadie - look good - I've made some suggestions on handling the construct-doesn't exist case.

[sadielbartholomew]

Thanks for your feedback @davidhassell. I'll update the PR shortly according to your excellent suggestions, though I will get the halo PR review in first, so do it after that.

[sadielbartholomew]

All ready for your re-review, @davidhassell. Thanks.

[davidhassell]

Look great. Thanks!

[sadielbartholomew]

Thanks David. In that case, merging!