Jacobi iteration: data structures
MPI_Type_contiguous (ypoints, MPI_DOUBLE, &contig);
MPI_Type_vector (xpoints, 1, ypoints+2, MPI_DOUBLE, &strided);
MPI_Type_commit (&contig);
MPI_Type_commit (&strided);
/* define mask array to be true on boundary and false elsewhere in
Define boundary values in phi array in each processor.