Copy additional coordinates when regridding

[bouweandela]

Description

The regrid preprocessor function regrids one set of coordinates to another. However, CORDEX data typically has two coordinates: rotated_latitude/rotated_longitude or projection_y_coordinate/projection_x_coordinate as a iris.coords.DimCoord and latitude/longitude as an additional iris.coords.AuxCoord. The changes in this pull request ensure that if the target grid has multiple coordinates, these will be available on the regridded cube, even if only one set is used for the regridding.

Closes #2839

Link to documentation:

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[codecov]

Codecov Report

❌ Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.13%. Comparing base (df77d1b) to head (8d24d14).

Files with missing lines	Patch %	Lines
esmvalcore/preprocessor/_regrid.py	86.20%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3048      +/-   ##
==========================================
- Coverage   96.15%   96.13%   -0.03%     
==========================================
  Files         270      270              
  Lines       15805    15822      +17     
==========================================
+ Hits        15197    15210      +13     
- Misses        608      612       +4     

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