Fix variable declaration for divV in post.cpp

[dlansigan]

Current situation

Resolves #455

Release Notes

• Remove second declaration of divV, which was causing divergence=0.0 bug for 3D fluid simulations.
• Updated reference files for tests with new divergences.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.60%. Comparing base (2f17399) to head (8f38977).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #484      +/-   ##
==========================================
+ Coverage   67.52%   67.60%   +0.07%     
==========================================
  Files         169      169              
  Lines       32671    32673       +2     
  Branches     5728     5729       +1     
==========================================
+ Hits        22062    22087      +25     
+ Misses      10472    10447      -25     
- Partials      137      139       +2     

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[aabrown100-git]

Looks good to me!

[aabrown100-git]

@dlansigan Before closing and merging, could you create another issue about the actual divergence values and whether they are expected (as I mentioned here)