[misc] minor performance & memory improvements

[felixdittrich92]

This PR:

• multiple after conversion saves some memory instead reverse
• use scipy connected_components instead of python loop
• tests: fix minor typo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (247750a) to head (2e9c5b7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2120      +/-   ##
==========================================
+ Coverage   96.96%   96.98%   +0.01%     
==========================================
  Files         169      169              
  Lines        9619     9611       -8     
==========================================
- Hits         9327     9321       -6     
+ Misses        292      290       -2     

Flag	Coverage Δ
unittests	96.98% <100.00%> (+0.01%)	⬆️

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