[ROS2][feat] Always use multilidar

Goal

This MR has the same purpose as MR !272 (merged)

• Trigger the SLAM process only when all LiDARs are received.
• For now, one LiDAR sensor was considered as master and the others were added when available.
• Most of the times, when various Lidars are used, it is to solve some missing degree of liberty in the independent LiDAR fields of view. So, if we allow the SLAM to process one only LiDAR frame (because it is all that it has at some point), it can fail and never recover.
• Now, the SLAM outputs a pose each time it has gathered all frames (it will be based on the sensor with the lowest frequency)
• A time parameter allows to drop frames if some have never been received

Changes

• Add parameters MultiLidarsNum to indicate the number of LiDAR sensors used
• Refact ScanCallback function to gather all frames from different LiDAR sensors
• Remove SecondaryScanCallback function

Results

This is tested on a robosense multilidar data.

With the new changes in feat/ROS2, please see the following commands:

• Change parameters in slam_config_indoor.yaml

lidars_nb: 2
input: 
  - "lidar_points_1"
  - "lidar_points_2"

• In slam_velodyne.py, add tf of two lidars:

  # left lidar link
  tf_left_node = Node(package="tf2_ros", executable="static_transform_publisher", name="tf_left_to_base",
    arguments=["--x", "-0.655", "--y", "0.455", "--z", "0.364",
               "--roll", "0", "--pitch", "0", "--yaw", "2.356",
               "--frame-id", "base_link", "--child-frame-id", "bpearl_back_left_frame"],
  )

  # right lidar link
  tf_right_node = Node(package="tf2_ros", executable="static_transform_publisher", name="tf_right_to_base",
    arguments=["--x", "0.655", "--y", "-0.455", "--z", "0.364",
               "--roll", "0", "--pitch", "0", "--yaw", "-0.785",
               "--frame-id", "base_link", "--child-frame-id", "bpearl_front_right_frame"],
  )

comment this line:  
# ld.add_action(velodyne_conversion_node)  
add:
  ld.add_action(tf_left_node)
  ld.add_action(tf_right_node)

run in two terminals robosense_conversion_node:

ros2 run lidar_conversions robosense_conversion_node --ros-args --remap /lidar_points:=/lidar_points_1 --ros-args --remap /rslidar_points:=/lidars/bpearl_back_left
ros2 run lidar_conversions robosense_conversion_node --ros-args --remap /lidar_points:=/lidar_points_2 --ros-args --remap /rslidar_points:=/lidars/bpearl_front_right

launch slam

ros2 launch lidar_slam slam_velodyne.py outdoor:=false

play rosbag

ros2 bag play path/to/rosbag  --clock -d 1 --topics /lidars/bpearl_back_left /lidars/bpearl_front_right

TODO

• Clean
• Reenable removed features from previously master callback
• Make it NLidars compatible (only two for now)
• Update MR !272 (merged) when this MR is done