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

@@            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Superseded.