Add dummy inertias for Gazebo simulation

[eborghi10]

Inertias are required to launch a Gazebo simulation using the URDF files from microstrain_inertial_description. This code can also be added to your robot:

<!-- Parker IMU -->
<gazebo reference="parker_center_joint">
  <preserveFixedJoint>true</preserveFixedJoint>
</gazebo>
<gazebo reference="parker_joint">
  <preserveFixedJoint>true</preserveFixedJoint>
</gazebo>
<gazebo reference="parker">
  <sensor name="parker_imu" type="imu">
    <pose>0 0 0 0 0 0</pose>
    <always_on>true</always_on>
    <update_rate>0</update_rate>
    <visualize>false</visualize>
    <plugin filename="libgazebo_ros_imu_sensor.so" name="imu_plugin">
      <topicName>/parker/imu</topicName>
      <bodyName>parker</bodyName>
      <updateRateHZ>50.0</updateRateHZ>
      <xyzOffset>0 0 0</xyzOffset>
      <gaussianNoise>0</gaussianNoise>
      <rpyOffset>0 0 0</rpyOffset>
      <frameName>parker</frameName>
      <initialOrientationAsReference>false</initialOrientationAsReference>
    </plugin>
  </sensor>
</gazebo>