[RFC] Add nbstripout to pre-commit hooks

[akihironitta]

What does this PR do?

Adds nbstripout to pre-commit hooks to remove outputs in notebooks.
https://github.com/kynan/nbstripout

Before submitting

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• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes?
• [n/a] Did you write any new necessary tests? [not needed for typos/docs]
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[codecov]

Codecov Report

Merging #336 (088904b) into master (660438f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #336   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          80       80           
  Lines        4061     4061           
=======================================
  Hits         3598     3598           
  Misses        463      463           

Flag	Coverage Δ
unittests	88.59% <ø> (ø)

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