fix_iris_issue

[rhaegar325]

Promote 2D latitude/longitude from data vars to xarray coords so ncdata converts them to iris AuxCoords instead of separate Cubes

Convert CMIP6 fill values (1e20) to NaN → masked arrays so iris properly masks invalid ocean cells

Promote vertices_latitude/vertices_longitude to coords so bounds are preserved on the auxiliary coordinates

Drop time_bnds before conversion to avoid extra cubes

Return a single Cube instead of a CubeList

Add 14 unit tests covering aux coords, masking, bounds, atmosphere data, edge cases, and dataset immutability

fixed #192

[codecov]

Codecov Report

❌ Patch coverage is 0% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 59.30%. Comparing base (1bdd2de) to head (3ff35dd).

Files with missing lines	Patch %	Lines
src/access_moppy/driver.py	0.00%	31 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #193      +/-   ##
==========================================
- Coverage   59.58%   59.30%   -0.29%     
==========================================
  Files          19       19              
  Lines        3125     3155      +30     
==========================================
+ Hits         1862     1871       +9     
- Misses       1263     1284      +21     

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