default invalid value is not sufficinet in Grid::Pos2Sub

[fangyuedong]

the code is

Eigen::Vector3i Pos2Sub(const Eigen::Vector3d& pos) const
{
      Eigen::Vector3i sub(0, 0, 0);
      for (int i = 0; i < dimension_; i++)
      {
          sub(i) = pos(i) - origin_(i) > -1e-7 ? static_cast<int>((pos(i) - origin_(i)) * resolution_inv_(i)) : -1;
      }
      return sub;
}

but sometimes the code do inflation as in ScanHandler::SetCurrentScanCloud, which makes invalid index become valid.

    Eigen::Vector3i c_sub = voxel_grids_->Pos2Sub(Eigen::Vector3d(point.x, point.y, point.z));
    for (int i = -L; i <= L; i++) {
        for (int j = -L; j <= L; j++) {
            for (int k = -N; k <= N; k++) {
                Eigen::Vector3i sub;
                sub.x() = c_sub.x() + i, sub.y() = c_sub.y() + j, sub.z() = c_sub.z() + k;
                if (voxel_grids_->InRange(sub)) {
                    const int ind = voxel_grids_->Sub2Ind(sub);
                    voxel_grids_->GetCell(ind) = voxel_grids_->GetCell(ind) | SCAN_BIT;
                }
            }
        }
    }

we should let sub(0..2)=-1000 to fix it.