[ROS][feat] Add aggregation node

• For now, the SLAM was outputting a pose and a map at each timestamp but not the aggregated frames
• An optional node aggregation is added to package lidar_slam to aggregate the points using registered Lidar scans outputted by lidar_slam_node
• A voxel grid is used to downsample the cloud.
• The new node has 3 parameters :
  • leaf_size : size of a voxel
  • max_size : max width of the voxel grid and so of the output cloud
  • min_points_per_voxel : min number of frames per voxel to consider the voxel contains a static object (moving objects rejection)
• A service is supplied to save the pointcloud on disk
• The new node can be run using the launch files with a parameter