message: demo_multi-session_mapping.launch: added rtabmap_args for convenience
diff: <!-- Choose visualization -->
  <arg name="rviz" default="false" />
  <arg name="rtabmapviz" default="true" />
  + <arg name="rtabmap_args" default="" />
  <param name="use_sim_time" type="bool" value="True"/>
  <!-- SLAM (robot side) -->
  <!-- args: "delete_db_on_start" and "udebug" -->
  - <node name="rtabmap" pkg="rtabmap_ros" type="rtabmap" output="screen" args="">
  + <node name="rtabmap" pkg="rtabmap_ros" type="rtabmap" output="screen" args="$(arg rtabmap_args)">
  <param name="frame_id" type="string" value="base_footprint"/>
  <param name="subscribe_depth" type="bool" value="true"/>
  