Introduce embedding mixin class

[Marius1311]

Besides simple joint pca, this now also implements a fast version of CCA to provide a good fallback for when no pre-computed representation is present.

closes #28

[codecov]

Codecov Report

❌ Patch coverage is 94.66667% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.67%. Comparing base (15d22f0) to head (0daa482).
⚠️ Report is 171 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cellmapper/model/embedding.py	89.79%	5 Missing ⚠️
src/cellmapper/utils.py	97.14%	2 Missing ⚠️
src/cellmapper/model/cellmapper.py	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
+ Coverage   84.05%   85.67%   +1.61%     
==========================================
  Files           7        9       +2     
  Lines         621      747     +126     
==========================================
+ Hits          522      640     +118     
- Misses         99      107       +8     

Files with missing lines	Coverage Δ
src/cellmapper/__init__.py	100.00% <100.00%> (ø)
src/cellmapper/check.py	100.00% <ø> (ø)
src/cellmapper/constants.py	100.00% <100.00%> (ø)
src/cellmapper/model/evaluate.py	80.00% <100.00%> (ø)
src/cellmapper/model/knn.py	79.76% <100.00%> (ø)
src/cellmapper/model/cellmapper.py	87.42% <95.45%> (ø)
src/cellmapper/utils.py	91.36% <97.14%> (+5.85%)	⬆️
src/cellmapper/model/embedding.py	89.79% <89.79%> (ø)

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