message: support robot ips as launch arg in combined control launch
diff: <!-- The config file containing all parameters for the combined hardware node like the IPs of the robots etc. -->
  <arg name="hw_config_file" default="$(find franka_control)/config/franka_combined_control_node.yaml"/>
  + <!-- Optional arg to manually configure the ips of all robots, overwriting ips that are configured in hw_config_file -->
  + <!-- The ips must be set as dictionary like {<arm_id_1>/robot_ip: <my_ip_1>, <arm_id_2>/robot_ip: <my_ip_2>} -->
  + <arg name="robot_ips" default="{}" />
  +
  <!-- The config file containing the parameterization for all controllers to start with this launch file -->
  <arg name="controllers_file" default="$(find franka_control)/config/default_combined_controllers.yaml" />
  <node name="$(arg robot_id)" pkg="franka_control" type="franka_combined_control_node" output="screen" required="true" >
  <rosparam command="load" file="$(arg hw_config_file)" />
  + <!-- Add or overwrite manually configured ips -->
  + <rosparam subst_value="True">$(arg robot_ips)</rosparam>
  <param name="robot_description" command="xacro --inorder $(arg robot) $(arg args)" />
  </node>
  