Made set_up_variable_cube test helper support non-square grid cells

[MO-PeterJordan]

Made set_up_variable_cube test helper support non-square grid cells
This supports making gradient divide by distance instead of degrees. Specifically it is required to allow testing of gradient for circular coordinate systems.

Testing:

• Ran tests and they passed OK
• Added new tests for the new feature(s) - n/a. Amended existing tests to cover new behaviour.

CLA

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[MoseleyS]

One small tweak should fix Sphinx.

Add your name and email address to .mailmap, this should fix the Contributing check.

[MO-PeterJordan]

One small tweak should fix Sphinx.

Add your name and email address to .mailmap, this should fix the Contributing check.

Done

[MO-PeterJordan]

Have rebased to edit original commit author to match correct name in mailmap (i.e. Peter Jordan)

[MoseleyS]

This still isn't happy. It seems that the commit that squashes all your previous commits together is authored by peter.jordan rather than MO-PeterJordan (use git log).
The CI Test uses git shortlog -s which shows that there is just this one commit to sort out. You might need to create a new branch with the right user, then cherry-pick this one commit into it and create a new PR from it.

[MoseleyS]

While all the automated CI tests pass, these do not include the acceptance tests, and there's one set that fail. They're all associated with this CLI:
improver/cli/generate_metadata_cube.py
The issue is that the cli always passes in a keyword argument {"grid_spacing": None}, except for the one acceptance test that actually sets a value to it. This code needs reworking to get it to pass.

source etc/site-init
export IMPROVER_ACC_TEST_DIR=/data/users/cfst/improver_test_data/data
pytest improver_tests/acceptance/test_generate_metadata_cube.py

[MO-PeterJordan]

All tests now pass. Ready for re-review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (96cdbef) to head (d50ef06).

❗ Current head d50ef06 differs from pull request most recent head 02c284b. Consider uploading reports for the commit 02c284b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1988   +/-   ##
=======================================
  Coverage   98.39%   98.39%           
=======================================
  Files         124      124           
  Lines       12209    12212    +3     
=======================================
+ Hits        12013    12016    +3     
  Misses        196      196           

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[mspelman07]

Thanks Peter, generally I think the changes look good, just a couple of small things to look at.

[MoseleyS]

Re-approved