Enforcing clearing of the notebook output

[christian-oreilly]

As this project will rely a lot on notebooks and notebooks are notoriously bad for doing git diffs, we need to implement coding rules that make their review manageable. Here is a comment from @scott-huberty that I think is worth investigating and implementing:

I came across an interesting idea for managing Jupyter notebooks in Git.

Use a pre-commit hook* to clear the output of the jupyter notebooks in your repository each time you make a commit. Something like:

jupyter nbconvert --clear-output --inplace .ipynb

That way, git only has to deal with the git diff for actual changes to the jupyter notebook python/markdown cells, not their output.

*Pre-commit is a DevOps python package for doing things like automatically enforcing style/linting rules before comitting. If the pre-commit fails, it will prevent you from making the commit. MNE-Python usees it so that contributors/devs follow basic style guidelines before opening up a PR (i.e. you don't have to dedicate an entire CI to catching flakes).

further reading on pre-commit: https://stefaniemolin.com/articles/devx/pre-commit/behind-the-scenes/

@bcalford Could you have a look into this when convenient?