topology_check_resort inconsistent type

[hirschsn]

Introduced in 7f4caf7, topology_check_resort returns true as default. This was used in cells_update_ghosts to decide if resorting is necessary or not. Later in 4e468e9, the function was changed to return an unsigned and is currently defined as follows:

unsigned topology_check_resort(int cs, unsigned local_resort) {
  switch (cs) {
  case CELL_STRUCTURE_DOMDEC:
  case CELL_STRUCTURE_NSQUARE:
    return boost::mpi::all_reduce(comm_cart, local_resort,
                                  std::bit_or<unsigned>());
  default:
    return true; // <----
  }
}

Results should semantically be values of enum Cells::Resort.

Now I don't directly do a PR because I don't know what you @fweik intend to be returned here: Cells::Resort::LOCAL or Cells::Resort::GLOBAL. In my opinion it should be GLOBAL. Because if this case would ever happen, we need to return a value v s.t. v >= local_resort on all processes. And this we can guarantee only for GLOBAL.

[fweik]

Yes, I agree it should be GLOBAL. The implicit integer conversion rules are great stuff ^^