It should be clear from this example that the Jacobi iteration is a very slow
method.  
Because this iteration converges so
slowly, it is necessary to test for convergence less frequently.
<P>
It is possible to avoid the step that copies xnew back into xlocal either by
using pointer operations (basically swapping xnew and xlocal) or by doing two
iterations at a time.  This latter approach requires another exchange of ghost
points and another relaxation sweep.  
