Returns the part of the matrix associated with the on-process coupling
PetscErrorCode MatGetDiagonalBlock(Mat A,Mat *a)
|a ||- the diagonal part (which is a SEQUENTIAL matrix)
see the manual page for MatCreateAIJ() for more information on the "diagonal part" of the matrix.
Use caution, as the reference count on the returned matrix is not incremented and it is used as
part of the containing MPI Mat's normal operation.