An extremely simple quadtree implementation. This implementation emphasizes
simplicity and correctness over performance, to serve as a base for an
understanding of the other implementations.

The tree has the following properties (or lack thereof):

• No attempt is made to distinguish between static and movable objects.
  In most games and other simulations, objects are inserted into an empty
  quadtree once per frame. If most of those objects do not move (such as
  static geometry in the game/simulation world), then they will always be
  inserted into the same point in the tree and re-inserting them every frame
  is wasteful and/or redundant. An obvious optimization, therefore, is to
  distinguish between static and movable objects in the tree and to provide
  support for removing just the movable objects in one efficient operation.


• 
  As various operations can potentially traverse the entire tree, it is
  desirable to keep the number of nodes (quadrants) in the tree to a minimum.
  This implementation does not allow the programmer to specify a minimum size
  for quadrants and does not attempt to remove redundant nodes when an object
  is removed from the tree.


• 
  Each child quadrant of a given quadrant is exactly half the width and
  height of the parent.


• 
  Values of type QuadTreeBasic cannot be safely accessed from
  multiple threads without explicit synchronization.

@param The type of objects contained within the tree.
@author Mark Raynsford

Construct a new quadtree with the given size and position.
param  size
The size.
param  position
The position.
return  A new quadtree.
param  
The precise type of quadtree members.