Fix bug + more robust tests

[gdalle]

Source

• In star coloring, initialize first_neighbors by filling it with (0, 0)

Tests

• Add more random tests so that bugs will be caught with higher probability (the one introduced by Working star coloring #11 was not detected in the PR CI, only on main)

Docs

• Correct installation instruction in the README

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.75%. Comparing base (759ed65) to head (5997a9e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   97.55%   97.75%   +0.20%     
==========================================
  Files           8        9       +1     
  Lines         286      312      +26     
==========================================
+ Hits          279      305      +26     
  Misses          7        7              

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