message: ex1p: implement pa jacobi preconditioning
  Performance is disappointing at the moment.
diff: @@ -208,9 +208,18 @@ int main(int argc, char *argv[])
  // 13. Solve the linear system A X = B.
  // * With full assembly, use the BoomerAMG preconditioner from hypre.
  - // * With partial assembly, use no preconditioner, for now.
  + // * With partial assembly, use jacobi smoothing, for now.
  Solver *prec = NULL;
  - if (!pa) { prec = new HypreBoomerAMG; }
  + if (pa)
  + {
  + Vector diag_pa(fespace->GetTrueVSize());
  + a->AssembleDiagonal(diag_pa);
  + prec = new VectorSmoother(diag_pa, ess_tdof_list);
  + }
  + else
  + {
  + prec = new HypreBoomerAMG;
  + }
  CGSolver cg(MPI_COMM_WORLD);
  cg.SetRelTol(1e-12);
  cg.SetMaxIter(2000);
  