Work with pre-computed objects for imputed values and k-NN graphs

[Marius1311]

First, this PR makes CellMapper more versatile as an evaluation tool for expression imputation computed externally. Second, it allows importing externally computed k-NN graphs (we test for scanpy and squidpy) via a new self-mapping mode. This can be useful e.g. for spatial smoothing, like in UTAG.

[codecov]

Codecov Report

❌ Patch coverage is 87.16578% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.17%. Comparing base (f0fb6eb) to head (5fb88f1).
⚠️ Report is 201 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cellmapper/utils.py	85.50%	10 Missing ⚠️
src/cellmapper/cellmapper.py	82.69%	9 Missing ⚠️
src/cellmapper/knn.py	92.98%	4 Missing ⚠️
src/cellmapper/evaluate.py	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   84.48%   85.17%   +0.68%     
==========================================
  Files           6        7       +1     
  Lines         477      607     +130     
==========================================
+ Hits          403      517     +114     
- Misses         74       90      +16     

Files with missing lines	Coverage Δ
src/cellmapper/check.py	100.00% <ø> (ø)
src/cellmapper/evaluate.py	84.45% <88.88%> (-0.21%)	⬇️
src/cellmapper/knn.py	79.41% <92.98%> (+4.02%)	⬆️
src/cellmapper/cellmapper.py	87.71% <82.69%> (-0.12%)	⬇️
src/cellmapper/utils.py	85.50% <85.50%> (ø)

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