message: SetDiscreteTargetSize fix
diff: #if defined(MFEM_USE_MPI) && defined(MFEM_TMOP_MPI)
  extern mfem::MPI_Session *GlobalMPISession;
  #define PFesGetParMeshGetComm(pfes) pfes.GetParMesh()->GetComm()
  +#define SetDiscreteTargetSize SetParDiscreteTargetSize
  #else
  typedef int MPI_Session;
  #define ParMesh Mesh
  @@ -28,6 +29,7 @@ typedef int MPI_Session;
  #define GetParGridFunctionEnergy GetGridFunctionEnergy
  #define PFesGetParMeshGetComm(...)
  #define MPI_Allreduce(src,dst,...) *dst = *src
  +#define SetDiscreteTargetSize SetSerialDiscreteTargetSize
  #endif
  using namespace std;
  @@ -301,7 +303,7 @@ int tmop(int myid, Req &res, int argc, char *argv[])
  tc->SetAdaptivityEvaluator(new AdvectorCG);
  FunctionCoefficient ind_coeff(discrete_size_2d);
  size.ProjectCoefficient(ind_coeff);
  - tc->SetSerialDiscreteTargetSize(size);
  + tc->SetDiscreteTargetSize(size);
  target_c = tc;
  break;
  }
  