Add a pre-commit hook to clean notebook outputs

[jhlegarreta]

In order to try to avoid inadvertently committing and merging the output or result of the execution of the jupyter notebooks, we should incorporate a pre-commit hook that clears the output cells using for example the following command:

jupyter nbconvert --ClearOutputPreprocessor.enabled=True --clear-output *.ipynb

The above requires installing the nbconvert package. Other solutions that do not require any additional package might exist.

Cross-referencing for example #109.