message: add option skip_zeros=2 for skipping all zeros
diff: @@ -2473,7 +2473,7 @@ void SparseMatrix::AddSubMatrix(const Array<int> &rows, const Array<int> &cols,
  {
  // if the element is zero do not assemble it unless this breaks
  // the symmetric structure
  - if (&rows != &cols || subm(j, i) == 0.0)
  + if ((skip_zeros == 2) || (&rows != &cols || subm(j, i) == 0.0))
  {
  continue;
  }
  