Add support for Geostationary coord_system

[stephenworsley]

Closes #5485

Supports the cannonical coordinates of the Geostationary coord system (projection_x_angular_coordinate, projection_y_angular_coordinate) and their cannonical unit (radians).

[codecov]

Codecov Report

Attention: Patch coverage is 57.89474% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.72%. Comparing base (a47fd34) to head (5ed2426).
Report is 324 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/analysis/_regrid.py	20.00%	3 Missing and 1 partial ⚠️
lib/iris/fileformats/_nc_load_rules/actions.py	0.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #5745    +/-   ##
========================================
  Coverage   89.71%   89.72%            
========================================
  Files          90       91     +1     
  Lines       22815    22924   +109     
  Branches     5438     5462    +24     
========================================
+ Hits        20468    20568   +100     
- Misses       1617     1622     +5     
- Partials      730      734     +4     

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[trexfeathers]

Thanks @stephenworsley! It's presumably on your radar to add tests to cover each of these changes?

[stephenworsley]

@trexfeathers Yes that's the plan, I just need to figure out how to get the appropriate test data.

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