Add 'ns' argument to allow specifying the robotNamespace for Gazebo

The robotNamespace is conceptually different from arm_id.
While the latter just acts as a name prefix for links and joints,
the <robotNamespace> tag specifies the name of the ROS namespace, which is used
for key robot components like the joint_states topic, the controller_manager, etc.

To allow for an empty namespace, the default needs to be specified as empty.
Otherwise, i.e. having a non-empty default, specifying ns:="" on the commandline
will fall back to the default despite an (empty) value was provided.

franka_description/robots/dual_panda_example.urdf.xacro

@@ -26,11 +26,11 @@
   </link>
 
   <!-- right arm with gripper -->
-  <xacro:panda_arm arm_id="$(arg arm_id_1)" connected_to="base"  xyz="0 -0.5 1" safety_distance="0.03"/>
+  <xacro:panda_arm arm_id="$(arg arm_id_1)" ns="$(arg arm_id_1)" connected_to="base" xyz="0 -0.5 1" safety_distance="0.03" />
   <xacro:hand ns="$(arg arm_id_1)" rpy="0 0 ${-pi/4}" connected_to="$(arg arm_id_1)_link8" safety_distance="0.03"/>
 
   <!-- left arm with gripper -->
-  <xacro:panda_arm arm_id="$(arg arm_id_2)" connected_to="base"  xyz="0 0.5 1" safety_distance="0.03"/>
+  <xacro:panda_arm arm_id="$(arg arm_id_2)" ns="$(arg arm_id_2)" connected_to="base" xyz="0 0.5 1" safety_distance="0.03" />
   <xacro:hand ns="$(arg arm_id_2)" rpy="0 0 ${-pi/4}" connected_to="$(arg arm_id_2)_link8" safety_distance="0.03"/>
 
 </robot>

franka_description/robots/panda_arm.urdf.xacro

@@ -7,6 +7,8 @@
   <xacro:arg name="hand" default="false" />
   <!-- Is the robot being simulated in gazebo?" -->
   <xacro:arg name="gazebo" default="false" />
+  <!-- Namespace of robot in Gazebo/ROS (default: empty) -->
+  <xacro:arg name="ns" default="" />
 
   <xacro:unless value="$(arg gazebo)">
     <!-- Create a URDF for a real hardware -->
@@ -61,7 +63,7 @@
 
     <gazebo>
       <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
-        <robotNamespace>$(arg arm_id)</robotNamespace>
+        <robotNamespace>$(arg ns)</robotNamespace>
         <controlPeriod>0.001</controlPeriod>
         <robotSimType>franka_gazebo/FrankaHWSim</robotSimType>
       </plugin>

franka_gazebo/launch/panda.launch

@@ -43,6 +43,7 @@
                     gazebo:=true
                     hand:=$(arg use_gripper)
                     arm_id:=$(arg arm_id)
+                    ns:=$(arg arm_id)
                     xyz:='$(arg x) $(arg y) $(arg z)'
                     rpy:='$(arg roll) $(arg pitch) $(arg yaw)'">
     </param>