Replace :UL with :F to select a triangle

[gdalle]

• Replace the default value of uplo with :F, which is apparently common in numerical linear algebra.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.85%. Comparing base (0f00909) to head (0fb2f20).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   99.86%   99.85%   -0.01%     
==========================================
  Files          10       10              
  Lines         715      713       -2     
==========================================
- Hits          714      712       -2     
  Misses          1        1              

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