Clean up repo for Jupyter notebooks

Add ignore rule for notebook cache directory. Remove outputs, execution, count and other notebook JSON elements that cause spurious diffs. This is based on the automatic jquery cleaning at commit.

http://timstaley.co.uk/posts/making-git-and-jupyter-notebooks-play-nice/

requested review from @mdefende

Note, this requires users to set up the automatic filtering with git described in:

http://timstaley.co.uk/posts/making-git-and-jupyter-notebooks-play-nice/

approved this merge request

merged