Adding titles to table <td> elements.

This involved rewriting the _repr_html_ methods and adding a separate
_repr_html_ to ImageGrid to account for the coordinate differences. Also
mixed in here are changes to the __str__ methods of Block and Pixel
to better show the positions and to more adaptively display the class name
instead of having it hard coded.
Regenerating Notebooks so the tables contain the appropriate title attributes.
Adding tests specifically for the Pixel class.