Fix VTK reading bug

[kaipartmann]

See #151.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.32%. Comparing base (cc4e246) to head (31f0850).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #152      +/-   ##
==========================================
+ Coverage   90.28%   90.32%   +0.03%     
==========================================
  Files          43       43              
  Lines        4034     4040       +6     
==========================================
+ Hits         3642     3649       +7     
+ Misses        392      391       -1     

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