message: Fix and add a proper comment on the modification
diff: @@ -1347,7 +1347,9 @@ class Simplex_tree {
  });
  Filtration_value max_filt_border_value = filtration(*max_border);
  - if (simplex.second.filtration() < max_filt_border_value) {
  + // Replacing if(f<max) with if(!(f>=max)) would mean that if f is NaN, we replace it with the max of the children.
  + // That seems more useful than keeping NaN.
  + if (!(simplex.second.filtration() >= max_filt_border_value)) {
  // Store the filtration modification information
  modified = true;
  simplex.second.assign_filtration(max_filt_border_value);
  