pre-commit hook for clearing ipynb notebooks

[elbeejay]

Hopefully there's a pre-commit hook out there that auto-clears the .ipynb notebooks as well as their metadata so we avoid commiting cell outputs and other metadata (which may reveal sensitive information about local file structures).

[elbeejay]

Two candidates for this are:

1. https://github.com/kynan/nbstripout
2. https://gitlab.com/iamlikeme/nbhooks

Let me know if you have experience or a preference for either hook @g4brielvs.

[elbeejay]

I think we need to resolve this sooner rather than later because it is impacting @bpstewar's PRs #31 and #32 - I'll look into it now

[elbeejay]

Selected nbstripout as the other project is no longer actively maintained