Jacobi iteration with a 2D Array.

This code uses a 2-D blocked decomposition of a 2-d array with more
than one element per chare. The code runs for a specified number of
iterations, using a reduction to start each successive iteration.

A 5-point stencil pattern is used to update the value for each data
element. For example, the new value for element X is the current
value of X plus the current values of its left, right, top, and
bottom neighbors.

     T
   L X R
     B

X'  = (X + L + R + T + B) / 5.0

Boundary conditions are obeyed in this example.  Control is expressed
using structured dagger.
