Jacobi Iteration: send guard values
while (err > tol) { /* copy phi array to oldphi */
/* communicate edge rows and columns to neighboring processor to put in their guard rings */
/* Send right boundary to right neighbor
and receive left ghost vector in return from left neighbor*/ MPI_Sendrecv (&(oldphi[1][xpoints]), 1, strided, rightneighbor, 31, &(oldphi[1][0]), 1, strided, leftneighbor, 31, comm2d, &status);