Gdt40 save

[pp-mo]

Addresses #785

So far, so highly sketchy + experimental

NOTE: I don't really like 94a3d38 because we only want to support calling without coords for the sake of the existing unit tests.
But I tried adapting some of the existing mock-ist tests + it's really quite awkward

[scitools-ci]

Templating

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Templated files

The following changed files are templated:

• pyproject.toml, templated by SciTools/.github/templates/pyproject.toml

Footnotes

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   @scitools-templating: please no update notification on: iris-grib ↩

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.83333% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.83%. Comparing base (56bf257) to head (94a3d38).

Files with missing lines	Patch %	Lines
src/iris_grib/_save_rules.py	83.33%	8 Missing and 6 partials ⚠️
src/iris_grib/__init__.py	88.23%	1 Missing and 1 partial ⚠️
src/iris_grib/_grib2_convert.py	94.73%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #813      +/-   ##
==========================================
- Coverage   90.05%   89.83%   -0.23%     
==========================================
  Files          10       10              
  Lines        2564     2665     +101     
  Branches      420      438      +18     
==========================================
+ Hits         2309     2394      +85     
- Misses        156      165       +9     
- Partials       99      106       +7     

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