Consider add demo data for testing

[amgfernandes]

It's probably a good idea to add demo data and labels.

[pranjaldhole]

Maybe we move it to src/napari_labelling_assistant/_tests/test_data and and write a simple unit test for generating stats.

[codecov-commenter]

Codecov Report

Merging #13 (9569df6) into main (ca37b96) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #13   +/-   ##
=======================================
  Coverage   55.55%   55.55%           
=======================================
  Files           3        3           
  Lines         126      126           
=======================================
  Hits           70       70           
  Misses         56       56           

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