Added caching to prevent recomputing fixed matrices

[martinrohbeck]

Draft PR: do not merge yet.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 8.47458% with 108 lines in your changes missing coverage. Please review.

Project coverage is 20.98%. Comparing base (75da157) to head (d1e28e0).

Files with missing lines	Patch %	Lines
src/scdori/_core/train_grn.py	8.47%	108 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
- Coverage   21.49%   20.98%   -0.52%     
==========================================
  Files          26       26              
  Lines        1484     1568      +84     
==========================================
+ Hits          319      329      +10     
- Misses       1165     1239      +74     

Files with missing lines	Coverage Δ
src/scdori/_core/train_grn.py	9.49% <8.47%> (+0.96%)	⬆️

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