message: Fix deprecated function from PETSc 3.12
diff: #include "petscmathypre.h"
  #endif
  +// Backward compatibility
  +#if PETSC_VERSION_LT(3,12,0)
  +#define MatComputeOperator(A,B,C) MatComputeExplicitOperator(A,C)
  +#define MatComputeOperatorTranspose(A,B,C) MatComputeExplicitOperatorTranspose(A,C)
  +#endif
  +
  #include <fstream>
  #include <iomanip>
  #include <cmath>
  #include <cstdlib>
  +
  // Note: there are additional #include statements below.
  // Error handling
  @@ -1206,7 +1213,7 @@ void PetscParMatrix::ConvertOperator(MPI_Comm comm, const Operator &op, Mat* A,
  Mat B;
  PetscBool isaij;
  - ierr = MatComputeExplicitOperator(*A,&B); CCHKERRQ(comm,ierr);
  + ierr = MatComputeOperator(*A,MATMPIAIJ,&B); CCHKERRQ(comm,ierr);
  ierr = PetscObjectTypeCompare((PetscObject)B,MATMPIAIJ,&isaij);
  CCHKERRQ(comm,ierr);
  ierr = MatDestroy(A); CCHKERRQ(comm,ierr);
  