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 93.84%. Comparing base (379d296) to head (f546a91).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5178   +/-   ##
========================================
  Coverage    93.84%   93.84%           
========================================
  Files          182      182           
  Lines        22497    22497           
  Branches      3200     3200           
========================================
  Hits         21113    21113           
  Misses         922      922           
  Partials       462      462           

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[orbeckst]

Looks good to me. Thank you.