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.

---

📚 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[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: 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     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.