Add usermods and testmods directories for ww3

[alperaltuntas]

Description

This PR specifies the usermods and testmods file directories for WW3.

Checklist

• My code follows the style guidelines of this project (black formatting)
• I have performed a self-review of my own code
• My changes generate no new warnings
• I have added tests that exercise my feature/fix and existing tests continue to pass
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding additions and changes to the documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.47%. Comparing base (a80e567) to head (37d5018).
⚠️ Report is 113 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4979      +/-   ##
==========================================
- Coverage   31.49%   28.47%   -3.02%     
==========================================
  Files         264      262       -2     
  Lines       38869    38446     -423     
  Branches     8254     8138     -116     
==========================================
- Hits        12242    10948    -1294     
- Misses      25388    26237     +849     
- Partials     1239     1261      +22     

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