message: Check that user does not try to insert null_vertex in the complex.
diff: @@ -584,12 +584,14 @@ class Simplex_tree {
  std::pair<Simplex_handle, bool> res_insert;
  auto vi = simplex.begin();
  for (; vi != simplex.end() - 1; ++vi) {
  + GUDHI_CHECK(*vi != null_vertex(), "cannot use the dummy null_vertex() as a real vertex");
  res_insert = curr_sib->members_.emplace(*vi, Node(curr_sib, filtration));
  if (!(has_children(res_insert.first))) {
  res_insert.first->second.assign_children(new Siblings(curr_sib, *vi));
  }
  curr_sib = res_insert.first->second.children();
  }
  + GUDHI_CHECK(*vi != null_vertex(), "cannot use the dummy null_vertex() as a real vertex");
  res_insert = curr_sib->members_.emplace(*vi, Node(curr_sib, filtration));
  if (!res_insert.second) {
  // if already in the complex
  @@ -674,6 +676,10 @@ class Simplex_tree {
  // Copy before sorting
  std::vector<Vertex_handle> copy(first, last);
  std::sort(std::begin(copy), std::end(copy));
  + GUDHI_CHECK_code(
  + for (Vertex_handle v : copy)
  + GUDHI_CHECK(v != null_vertex(), "cannot use the dummy null_vertex() as a real vertex");
  + )
  std::vector<std::vector<Vertex_handle>> to_be_inserted;
  std::vector<std::vector<Vertex_handle>> to_be_propagated;
  