The .ipynb_checkpoints don't need to be on git

These dirs are created by IPython for restoring closed files. They don't
need to be commited to version control.
Reformatted the sphere-cube comparison notebook

Separated text from headers so that not all text is bold face. Inserted
anomaly figures, added a note about where these notebooks come from and
that they run Fatiando. I'll include this note on all other notebooks
from now on.