Extend tests & update the README

[Marius1311]

Increase coverage and improve the README

closes #14 #12 #6

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.23%. Comparing base (422d688) to head (fb3f9e9).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cellmapper/knn.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   78.25%   84.23%   +5.98%     
==========================================
  Files           6        6              
  Lines         423      425       +2     
==========================================
+ Hits          331      358      +27     
+ Misses         92       67      -25     

Files with missing lines	Coverage Δ
src/cellmapper/check.py	100.00% <100.00%> (+36.00%)	⬆️
src/cellmapper/knn.py	75.38% <0.00%> (+3.84%)	⬆️

... and 1 file with indirect coverage changes

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