message: Update test_velodyne.launch
diff: <param name="approx_sync" type="bool" value="false"/>
  <remap from="scan_cloud" to="assembled_cloud"/>
  + <remap from="imu" to="$(arg imu_topic)"/>
  <!-- RTAB-Map's parameters -->
  <param name="Rtabmap/DetectionRate" type="string" value="1"/>
  <param name="Grid/RangeMax" type="string" value="20"/>
  <param name="Grid/ClusterRadius" type="string" value="1"/>
  <param name="Grid/GroundIsObstacle" type="string" value="true"/>
  + <param name="Optimizer/GravitySigma" type="string" value="0.3"/>
  <!-- ICP parameters -->
  <param name="Icp/VoxelSize" type="string" value="0.2"/>
  