Field values should return a copy

[sandorkertesz]

Description

With this PR Field.values always returns a copy.

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 56.62651% with 36 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.43%. Comparing base (a8d48bc) to head (eb33fb2).

Files with missing lines	Patch %	Lines
tests/netcdf/test_netcdf_values.py	10.00%	18 Missing ⚠️
tests/readers/test_geotiff_reader.py	14.28%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1009      +/-   ##
===========================================
- Coverage    81.56%   81.43%   -0.14%     
===========================================
  Files          236      236              
  Lines        15510    15592      +82     
  Branches       755      755              
===========================================
+ Hits         12651    12697      +46     
- Misses        2634     2670      +36     
  Partials       225      225              

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