NAME    pick_center - Interactive pick on scene to center for rotation.
 
 AUTHOR  Ian Curington, Advanced Visual Systems, UK
 
 DESCRIPTION
 
   This module allows the user to pick on anything in the GeometryViewer
   scene that is not already registered for upstream picking,
   and will set the current rotate / zoom center to be this picked point.
   It is most useful if you see a large complexe scene, pick on
   the piece you want to zoom into, then zoom into it. Without
   this module, if it was on the edge of the object it would disappear
   from view as soon as you zoomed in.
  
   The implementation:
    It creates a null geometry, to simply register communication
    with the GeomViewer.
    It has an upstream_geom port, notified on picks to "top".
    It then sends a CLI command to change the center to
    the picked position.