Add a usage message for toggling point movement

For #2
Optimize remove(...)

* Change remove(...) to only call _remove_empty_subtrees() if a point
was actually removed.
* Rename removed_point to point_was_removed.
Add Point.get_translated_point
Add PointQuadTree.translate_point

Also change the viewer to use it to move points.
Also change the viewer's random points to have a non-zero speed and
allow it to be fractional.