JNB: Blacken notebooks

[scottclowe]

Convert notebooks to the Black code style

Add a pre-commit hook to ensure notebooks always conform to the Black format.

[codecov]

Codecov Report

Merging #180 (cf17610) into master (fd9d282) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #180   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files           8        8           
  Lines         831      831           
  Branches      165      165           
=======================================
  Hits          770      770           
  Misses         31       31           
  Partials       30       30           

Flag	Coverage Δ
unittests	92.53% <ø> (ø)

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[scottclowe]

This didn't affect cells starting with cell magic, so I'm fixing that now.

[swkeemink]

This was my first time learning about the black code style, so I just wanted to link the main site about the style for PR clarity:
https://black.readthedocs.io/en/stable/

Otherwise very happy with this PR, should use this in future projects.