[VoxelGrid] Remove getLeafLayout

Is your feature request related to a problem? Please describe.
Raised while refactoring VoxelGrid https://github.com/PointCloudLibrary/pcl/pull/4829#pullrequestreview-698580970

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: https://github.com/PointCloudLibrary/pcl/issues/4365 https://github.com/PointCloudLibrary/pcl/issues/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>

Start with the VoxelGrid implementation and the refactoring context in pull request #4829, then review the linked issues #4365 and #585 for the size_t migration. Trace getLeafLayout(), getCentroidIndexAt(), leaf_layout_, and hashPoint() to determine the affected API and tests. Done means callers can query by point coordinate or leaf hash without the old large int vector.