Implement NonlinearReactionOperator.restricted

[sdrave]

@HenKlei, found this code in an old branch. Should we merge it (after testing it)?

[HenKlei]

@HenKlei, found this code in an old branch. Should we merge it (after testing it)?

Sounds good!

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.78%. Comparing base (225e12a) to head (675438f).
⚠️ Report is 765 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pymor/discretizers/builtin/fv.py	33.33%	4 Missing ⚠️

Additional details and impacted files

Flag	Coverage Δ
github_actions	78.46% <100.00%> (?)
gitlab_ci	86.81% <16.66%> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/pymor/discretizers/builtin/fv.py	71.48% <33.33%> (+4.23%)	⬆️

... and 171 files with indirect coverage changes

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