Jacobi iteration with a 2D Array.

Demonstrates 2d dense chare array creation and use.  This code computes
the steady state heat distribution of a 2d plate using the jacobi
iteration:

while new_temp != temp
  //next temperature is average of surrounding temperatures 
  new_temp(x,y) = (temp(x-1,y)+temp(x+1,y)+temp(x,y-1)+temp(x,y+1))/4
  temp(x,y) = new_temp(x,y)
end

Every temp(x,y) is a chare in this sample application.  The main chare
sends a notice to all the nodes in our simulation telling them to
exchange information.  After each node is done updating its
temperature, it sends a message back the error as the difference
between temp and new_temp.  Main quits driving the iterations when the
error has been minimized below some threshold.


