Updates docs and provides branchless GPU path

[tapios]

• Updates docs
• Provides option forced_fixed_iters that iterates for a fixed number of iterations.
• Verified that maxiter = 3 gives temperature accuracy better than 0.1 K (typically 1e-5 K).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.45%. Comparing base (979e344) to head (d81f006).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #297      +/-   ##
==========================================
+ Coverage   86.29%   86.45%   +0.15%     
==========================================
  Files          22       22              
  Lines        1102     1115      +13     
==========================================
+ Hits          951      964      +13     
  Misses        151      151              

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