[VoxelGrid] Remove getLeafLayout

[tin1254]

Is your feature request related to a problem? Please describe.
Raised while refactoring VoxelGrid #4829 (review)

As we are upgrading point hash datatype from int to size_t, the API std::vector<int> getLeafLayout() is not compatible anymore

Context

Reasons to upgrade to size_t: #4365 #585

Expected behavior

Users can query the voxel index using point coordinate or point hash

Current Behavior

We can query the voxel index using point coordinate with int getCentroidIndexAt(const Eigen::Vector3i&).
But if we wanted to query using point hash, we will need to first get a large vector with std::vector<int> getLeafLayout(), and query with the vector

Describe the solution you'd like

Use the following functions to replace getLeafLayout() to avoid return large vector when we upgrade to size_t.
leaf_layout_ will be a unordered_map

// int -> size_t
size_t getCentroidIndexAt(const Eigen::Vector3i& pt) { return leaf_layout_[hashPoint(pt)]; }
// new function
size_t getCentroidIndexAtLeafIdx(const size_t idx) { return leaf_layout_[idx]; }

Describe alternatives you've considered

Same as above, but leaf_layout_ is avector<size_t>

[kunaltyagi]

For the index, we'd need to let the user find the size as well

[tin1254]

you mean the number of voxels in the filtered cloud? but #voxels = #points in the filtered cloud

[kunaltyagi]

#voxels = #points in the filtered cloud

That's not necessarily true. In the new voxelgrid, you're making the voxel 0, but still not removing it from the map

[tin1254]

I mean use can for example get the size from the number of points in the output cloud

PointCloud<PointXYZ> output;
VoxelGrid<PointXYZ> grid;

grid.setLeafSize (0.02f, 0.02f, 0.02f);
grid.setInputCloud (cloud);
grid.filter (output);

Because the leaf_layout_ size is only incremented if a point is pushed to the output

inline experimental::optional<PointT>
filterGrid(const iterator grid_it)
{
  const auto& voxel = grid_it->second;
  if (voxel.size() >= min_points_per_voxel_) {

    if (save_leaf_layout_)
      leaf_layout_[grid_it->first] = num_voxels_++;

    return voxel.get();
  }

  return boost::none;
}

But anyway we can do that easily by returning the following member in the new voxelgrid

/** \brief Total number of voxels in the output cloud */
std::size_t num_voxels_;

[kunaltyagi]

https://github.com/tin1254/pcl/pull/2/files#diff-d07ab844703ea9f1f13139fe03f22f7399e31d1e603b128957659e7cce53bc4cR221-R226

This clearly leaves some leaves which are not added during the filter operation