Making attr_key required for GraphArrayView

[yfukai]

The default value of attr_key in GraphArrayView was bbox, which hardly made sense considering that the value should be a scalar value to display in the mask. It may make more sense to make this required and also check if the prop is a scalar value.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.96%. Comparing base (a188502) to head (bfd77e0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   87.95%   87.96%   +0.01%     
==========================================
  Files          50       50              
  Lines        3411     3416       +5     
  Branches      585      586       +1     
==========================================
+ Hits         3000     3005       +5     
  Misses        247      247              
  Partials      164      164              

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[JoOkuma]

Hi @yfukai, thanks for fixing this issue and adding the regression tests.
I don't know how it went through.
I made a single change to fix a missing argument type.

[yfukai]

Thanks for fixing the PR and merging! @JoOkuma