message: Update demo_turtlebot_mapping.launch
  Min grid size set to 20 meters for convenience
diff: <param name="Mem/RehearsalSimilarity" type="string" value="0.30"/>
  <param name="Optimizer/Slam2D" type="string" value="true"/>
  <param name="Reg/Force3DoF" type="string" value="true"/>
  + <param name="GridGlobal/MinSize" type="string" value="20"/>
  <!-- localization mode -->
  <param if="$(arg localization)" name="Mem/IncrementalMemory" type="string" value="false"/>
  