- _index is used for partial (re)indexing during 2D (multi-threading) and 3D (initial mesh) and is thus less
  susceptible to go > 2 billion
- _index is used in IO formats that don't support 64 bit indices anyway

Eventually _index should be removed; we should store a table of correspondance with _num on-the-fly where
and when necessary. This will be done after the transition to size_t for num.