Normalise units of coordinate bounds

[bjlittle]

🚀 Pull Request

Description

This pull request ensures that during the NetCDF loading pipeline, coordinates are constructed with points and bounds equal to the units of the points.

This is to resolve the issue where coordinates can be constructed and attached to a cube, but the units of the points and bounds of the coordinate are encoded differently within the source NetCDF file.

The following cases are handled:

1. Bounds that have invalid units i.e., not recognised by cf-units, will result in a warning being raised and the bounds attached "as-is" to the coordinate. This provides the user to option to keep, remove or replace the bounds as they choose.
2. Bounds with units that are different but convertible to the units of the points will be automatically converted to the units of the points.
3. Bounds with units that are different and not convertible to the units of the points will result in a warning being raised and the bounds being ignored i.e., not attached to the coordinate.
4. Bounds with no associated NetCDF metadata specifying their units will be assumed to have the same units of the points i.e., they will be attached to the coordinate "as-is".

Note that, the test_build_auxiliary_coordinate.TestBoundsVertexDim and test_build_dimension_coordinate.TestBoundsVertexDim tests have been extended to include test coverage of converting bounds to the units of the points, [2] above. This is to avoid heavy replication of boiler-plate mocking infrastructure.

TODO:

• Add whatsnew entry.

Finally, this one's for you @andreas-h.

Closes #1801

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[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a47fd34) 89.71% compared to head (35a629f) 89.74%.
Report is 14 commits behind head on main.

Files	Patch %	Lines
lib/iris/fileformats/_nc_load_rules/helpers.py	80.95%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5746      +/-   ##
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+ Coverage   89.71%   89.74%   +0.02%     
==========================================
  Files          90       91       +1     
  Lines       22815    22925     +110     
  Branches     5438     5462      +24     
==========================================
+ Hits        20468    20573     +105     
- Misses       1617     1620       +3     
- Partials      730      732       +2     

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[trexfeathers]

Finally, this one's for you @andreas-h.

❤

[stephenworsley]

Looks good, just a small suggestion about warnings. This is a warning that already existed, so there may be a case for seperating it out into its own issue if changing it impacts a lot of the existing tests.

[stephenworsley]

Looks good

[bjlittle]

“Andreas Hilboll” Scholarship for researchers