Field grid_spec should return a copy

[sandorkertesz]

Description

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 27.27273% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.35%. Comparing base (e7a00c1) to head (162829c).

Files with missing lines	Patch %	Lines
tests/grib/test_grib_geography.py	27.27%	16 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1010      +/-   ##
===========================================
- Coverage    81.43%   81.35%   -0.08%     
===========================================
  Files          236      236              
  Lines        15592    15614      +22     
  Branches       755      755              
===========================================
+ Hits         12697    12703       +6     
- Misses        2670     2686      +16     
  Partials       225      225              

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