tests: modernize deprecated numpy assertions in test_gnm.py (ref #3743)

[shubhammittal2241]

Contributes to issue #3743.

This PR updates deprecated NumPy testing functions in test_gnm.py:

• Replaces assert_almost_equal and assert_array_almost_equal with numpy.testing.assert_allclose
• Uses pytest.approx for scalar floating point comparisons
• Preserves existing tolerances and test behavior
• All tests pass locally

Added myself to package/AUTHORS as a first-time contributor.

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📚 Documentation preview 📚: https://mdanalysis--5178.org.readthedocs.build/en/5178/

[shubhammittal2241]

This PR is ready for review.
All tests pass locally, and the only CI failure was a cancelled job due to duplicate workflow runs by GitHub.
@orbeckst @RMeli @marinegor — Please let me know if anything else is needed. Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.73%. Comparing base (bbcef1b) to head (bf6388b).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5178      +/-   ##
===========================================
+ Coverage    92.72%   92.73%   +0.01%     
===========================================
  Files          180      180              
  Lines        22472    22472              
  Branches      3188     3188              
===========================================
+ Hits         20837    20840       +3     
+ Misses        1177     1175       -2     
+ Partials       458      457       -1     

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