Convert max label to integer before using it as parameter for np.zeros

[stefanhahmann]

This PR fixes a the following exception:

which was raised after this workflow:

The consequence of the exception was that no cluster_ids_in_space layer was generated after making a freehand selection in the clusters plotter.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7a5286e) 76.39% compared to head (2efe878) 76.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #284   +/-   ##
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  Coverage   76.39%   76.39%           
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  Files          16       16           
  Lines        1860     1860           
=======================================
  Hits         1421     1421           
  Misses        439      439           

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[Cryaaa]

@stefanhahmann,
Thanks for catching this! Slightly weird that the label image is not stored as an integer array but possibly the numpy max function returns non integers by default..