Make difference module handle circular cubes

[MO-PeterJordan]

Split out from https://github.com/metoppv/improver/pull/1985 to support calculating gradients for cubes which wrap around the meridian.

Testing:

• [x] Ran tests and they passed OK
• [x] Added new tests for the new feature(s)

CLA

• [x] If a new developer, signed up to CLA

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.87179% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.41%. Comparing base (84a8944) to head (31fceb3). Report is 12 commits behind head on master.

Files	Patch %	Lines
improver/utilities/spatial.py	94.87%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1990 +/- ## ========================================== + Coverage 98.39% 98.41% +0.01% ========================================== Files 124 128 +4 Lines 12212 12482 +270 ========================================== + Hits 12016 12284 +268 - Misses 196 198 +2 ```

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[MoseleyS]

Superseded.