message: Adds the 'transmission' arg to the 'panda_arm.urdf.xacro' file
  This commit gives users the ability to set the hardware_interface that
  is used in the panda arm joints when loading the 'panda_arm.urdf.xacro'
  file.
diff: <?xml version='1.0' encoding='utf-8'?>
  <robot xmlns:xacro="http://www.ros.org/wiki/xacro" name="panda">
  - <!-- Name of this panda -->
  - <xacro:arg name="arm_id" default="panda" />
  - <!-- Should a franka_gripper be mounted at the flange?" -->
  - <xacro:arg name="hand" default="false" />
  - <!-- Is the robot being simulated in gazebo?" -->
  - <xacro:arg name="gazebo" default="false" />
  + <xacro:arg name="arm_id" default="panda" /> <!-- Name of this panda -->
  + <xacro:arg name="hand" default="false" /> <!-- Should a franka_gripper be mounted at the flange?" -->
  + <xacro:arg name="gazebo" default="false" /> <!-- Is the robot being simulated in gazebo?" -->
  + <xacro:arg name="transmission" default="hardware_interface/EffortJointInterface" /> <!-- The transmission that is used in the gazebo joints -->
  +
  + <xacro:property name="arm_id" value="$(arg arm_id)" />
  + <xacro:property name="transmission" value="$(arg transmission)" />
  <xacro:unless value="$(arg gazebo)">
  <!-- Create a URDF for a real hardware -->
  </joint>
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint1" transmission="hardware_interface/EffortJointInterface" />
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint2" transmission="hardware_interface/EffortJointInterface" />
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint3" transmission="hardware_interface/EffortJointInterface" />
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint4" transmission="hardware_interface/EffortJointInterface" />
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint5" transmission="hardware_interface/EffortJointInterface" />
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint6" transmission="hardware_interface/EffortJointInterface" />
  - <xacro:gazebo-joint joint="$(arg arm_id)_joint7" transmission="hardware_interface/EffortJointInterface" />
  + <xacro:gazebo-joint joint="${arm_id}_joint1" transmission="${transmission}" />
  + <xacro:gazebo-joint joint="${arm_id}_joint2" transmission="${transmission}" />
  + <xacro:gazebo-joint joint="${arm_id}_joint3" transmission="${transmission}" />
  + <xacro:gazebo-joint joint="${arm_id}_joint4" transmission="${transmission}" />
  + <xacro:gazebo-joint joint="${arm_id}_joint5" transmission="${transmission}" />
  + <xacro:gazebo-joint joint="${arm_id}_joint6" transmission="${transmission}" />
  + <xacro:gazebo-joint joint="${arm_id}_joint7" transmission="${transmission}" />
  <xacro:transmission-franka-state arm_id="$(arg arm_id)" />
  <xacro:transmission-franka-model arm_id="$(arg arm_id)"
  