Patch to work with opensim trunk ca. 2009/12/03.

 * prebuild patch updated
 
 * Biggest change: Physics engines now use Vector3 instead of
   PhysicsVector.  This was more than a search and replace, because
   Vector3 has a Multiply(Vector3, float) method, but *not* a
   Multiply(float, Vector3)... PhysicsVector had both.  So I had to
   rearrange some products.  Additionally, Vector3 is a struct whereas
   PhysicsVector was a class; it turns out that when you pass a struct
   in C#, it is passed by *value*, but a class is passed by
   *reference*.  (I'm sure somebody thought that was a good idea.)  So,
   a couple of functions (periodicBCs and one that moves characters
   around) had to be updated to deal with this.

I've connected this to a grid for the first time, and am not sure that
the boundary conditions are functioning quite as desired.  I will
continue to futz with it.