message: rtabmap.launch: added gen_scan option for convenience
diff: <arg name="scan_descriptor_topic" default="/scan_descriptor"/>
  <arg name="scan_cloud_max_points" default="0"/>
  <arg name="scan_cloud_filtered" default="false"/> <!-- use filtered cloud from icp_odometry for mapping -->
  + <arg name="gen_scan" default="false"/> <!-- only works with depth image and if not subscribing to scan topic-->
  <arg name="visual_odometry" default="true"/> <!-- Launch rtabmap visual odometry node -->
  <arg name="icp_odometry" default="false"/> <!-- Launch rtabmap icp odometry node -->
  <param name="map_frame_id" type="string" value="$(arg map_frame_id)"/>
  <param name="odom_frame_id" type="string" value="$(arg odom_frame_id)"/>
  <param name="publish_tf" type="bool" value="$(arg publish_tf_map)"/>
  + <param name="gen_scan" type="bool" value="$(arg gen_scan)"/>
  <param name="ground_truth_frame_id" type="string" value="$(arg ground_truth_frame_id)"/>
  <param name="ground_truth_base_frame_id" type="string" value="$(arg ground_truth_base_frame_id)"/>
  <param name="odom_tf_angular_variance" type="double" value="$(arg odom_tf_angular_variance)"/>
  