In the simple Jacobi loop, at the end of each iteration, there is an
MPI_Allreduce used to test for convergence.  This provides a synchronization
point in the program.  This example uses that synchronization point to allow
the use of the MPI_Rsend routine.
<P>
To do this, the matching receives (as MPI_Isend) must be posted before the
synchronization point and the sends (as MPI_Rsend) must be posted after the 
synchronization point.
