VoxelGrid on GPU

It would be nice to have a voxel grid filter that runs on the GPU, and is thus faster than the existing pcl::VoxelGrid (which runs on the CPU). The new GPU voxel grid should have the same filtering behaviour as pcl::VoxelGrid, but does not have to implement filter_field_name_ or save_leaf_layout_ for now. It could maybe make use of pcl::gpu::Octree, or stay closer to the implementation of pcl::VoxelGrid and use the sorting functions from thrust.