Simplify initializations in GMGPolar::setup

[julianlitz]

Refactors the operator initialization section of GMGPolar::setup() to improve clarity and separation of concerns.
Previously, a single loop over all levels used if/else branching to handle level-specific logic (finest, coarsest, intermediate), mixing smoother, direct solver, and residual initialization in one place. This made the control flow harder to follow.

Merge Request - GuideLine Checklist

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• Tests for new functionality has been added
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Checks by code reviewer(s):

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.28%. Comparing base (a5f97e6) to head (36b4014).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #187      +/-   ##
==========================================
+ Coverage   95.21%   95.28%   +0.06%     
==========================================
  Files          94       94              
  Lines        9322     9306      -16     
==========================================
- Hits         8876     8867       -9     
+ Misses        446      439       -7     

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