fix unused down_sample:voxel_m parameter

[KacperSynator]

The leaf size used for downsampling was using magic number instead of down_sample:voxel_m parameter from json config file, this change fixes it.

Additionally:

• The name was changed from kLeafSize to leafSize as k* variables are reserved for global/static constants, which json parameter is not as it is dynamically assigned.
• Type was changed to const double as the value is not changed later and pcl::octree::OctreePointCloud constructor takes double.