message: Return 0 when element is undefined
diff: @@ -157,7 +157,7 @@ const double& BlockMatrix::Elem (int i, int j) const
  if (IsZeroBlock(iblock, jblock))
  {
  - mfem_error("BlockMatrix::Elem");
  + return 0;
  }
  return static_cast<const SparseMatrix *>(Aij(iblock, jblock))->Elem(iloc, jloc);
  }
  