BUGS

 - a reload (by pressing the 'r' key) zooms out and makes the image too
   small
      - This behavior is hard coded into vtk: Pressing R calls ResetCamera().
        We might need to make the boxes 30% bigger?
      - This doesn't help at all because the camera's zoom is tenaciously adjusted to the model.
      - In vtk's sources (vtkCommand.h) I found out, that there happens to be a ResetCameraEvent.
        Triggering a zoom factor adjustment after this event, at least improves the behavior.
        Still pressing 'r' multiple times results in some strange shrinking effect ... Round-off error maybe?
      - Virtual functions are not hooked into python, so it is not possible to override OnKeyPress() 
        and customize behavior on key presses.

Probably several more.  This is why a test suite is needed...