Support purge_experiments() to safely clean up generated experiments.

[minghangli-uni]

closes #25

3 options are provided:

• branches: If list is not specified, all experiments under test_path will be purged.
• hard: Enables payu sweep --hard, otherwise, payu sweep is used.
• dry_run: Performs a dry run to simulate the purge without deleting any files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.41%. Comparing base (fe25baa) to head (151abfd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   95.98%   96.41%   +0.43%     
==========================================
  Files           5        5              
  Lines         224      251      +27     
  Branches       34       40       +6     
==========================================
+ Hits          215      242      +27     
  Misses          3        3              
  Partials        6        6              

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