Fix mode for second order

[gdalle]

Fix #204

Core

• Define mode for SecondOrder by taking the mode of the outer backend

Tests

• Add a benchmarking test for SecondOrder to avoid regressions

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.90%. Comparing base (023a8ea) to head (3ca2ed9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #205      +/-   ##
==========================================
- Coverage   96.39%   95.90%   -0.50%     
==========================================
  Files          65       15      -50     
  Lines        3553     1562    -1991     
==========================================
- Hits         3425     1498    -1927     
+ Misses        128       64      -64     

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