Notebook checkpoint files

[stuckyb]

Hi, all,

A quick request: Let's not add notebook checkpoint files to the repository. They don't add anything useful since they are basically just back-up copies of the notebook files, which should already be under version control.

I've removed the notebook checkpoint files that were already committed to the repository and I added a .gitignore entry so they won't show up any more when you run git status.

Thanks!