Add device_to_data coordinate conversion

This is useful for GUIs, where mouse-clicks have to be
interpreted in terms of data coordinates.
Add data_to_device

It turns out that this is the coordinate transformation you more
typically want for GUIs, because in choosing the item nearest the
clicked point, users will expect it to be based on distance on the
screen, not based on distance in data units.
Merge pull request #82 from timholy/master

Add device_to_data coordinate conversion