DrawTreeDiagram:

--------------------------------------------------------------------------
   This function creates and draws a tree diagram.
--------------------------------------------------------------------------
   Form:
   [q, sIndex] = DrawTreeDiagram( hFig, tree, position, buttonDownFunction )
--------------------------------------------------------------------------

   ------
   Inputs
   ------
   hFig               (1,:)  Figure handle
   tree               {}     Tree cell array
   position           (1,4)  [x y width height]
   buttonDownFunction (1,:)  String of the form  function( xxxxx, 
                             The tree block id is substituded after xxxxx, to 
                             get function( xxxxx, 1 )

   -------
   Outputs
   -------
   q                  (1,1)  Data structure with the graphical display tree data
   sIndex             {}     index into graphical tree

--------------------------------------------------------------------------