message: Fix nnz for views + add test to expose
diff: @@ -929,6 +929,7 @@ where
  /// algorithms, which are often linear in the number of non-zeros.
  pub fn nnz(&self) -> usize {
  self.indptr.last().unwrap().index_unchecked()
  + - self.indptr.first().unwrap().index_unchecked()
  }
  /// The density of the sparse matrix, defined as the number of non-zero
  @@ -2711,10 +2712,14 @@ mod test {
  fn middle_outer_views() {
  let size = 11;
  let csr: CsMat<f64> = CsMat::eye(size);
  - assert_eq!(csr.view().middle_outer_views(1, 3).shape(), (3, size));
  + let v = csr.view().middle_outer_views(1, 3);
  + assert_eq!(v.shape(), (3, size));
  + assert_eq!(v.nnz(), 3);
  let csc = csr.to_other_storage();
  - assert_eq!(csc.view().middle_outer_views(1, 3).shape(), (size, 3));
  + let v = csc.view().middle_outer_views(1, 3);
  + assert_eq!(v.shape(), (size, 3));
  + assert_eq!(v.nnz(), 3);
  }
  #[test]
  