Some bugs in using SDF extensions from URDF

[osrf-migration]

Original report (archived issue) by Steve Peters (Bitbucket: Steven Peters, GitHub: scpeters).

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The urdf_gazebo_extensions.cc test uses urdf_gazebo_extensions.urdf to demonstrate the functionality for inserting SDF extension tags into URDF files with the <gazebo> tag, as described in the description of pull request #225. There are some issues with this functionality that I noticed while reviewing pr #506, and I'm working on illustrating them with a test in branch urdf_gazebo_extensions_fix.

A summary of the changes so far in that branch:

• 36b9c67: remove joint23 since it creates a kinematic loop and makes the URDF invalid
• 1c19e26: fix some expectations for joint12 since it is looking for elements in the wrong place
• e07490a: add //joint/physics/ode/cfm and //joint/physics/ode/erp tags using the syntax described in pr #225 and expect the tags to be included in the same //joint/physics/ode block as the other extension tags, but it appears to be added in a duplicate //joint/physics/ode block, as shown below:

    <joint name='joint01' type='revolute'>
      <pose relative_to='link0'>0 0 -1 0 -0 1.5708</pose>
      <parent>link0</parent>
      <child>link1</child>
      <axis>
        <xyz>1 0 0</xyz>
        <limit>
          <lower>-1e+16</lower>
          <upper>1e+16</upper>
        </limit>
        <dynamics>
          <damping>1.1111</damping>
          <friction>2.2222</friction>
          <spring_reference>0.234</spring_reference>
          <spring_stiffness>0.567</spring_stiffness>
        </dynamics>
      </axis>
      <physics>
        <provide_feedback>1</provide_feedback>
        <ode>
          <implicit_spring_damper>0</implicit_spring_damper>
          <cfm_damping>0</cfm_damping>
          <fudge_factor>0.56789</fudge_factor>
          <limit>
            <cfm>123</cfm>
            <erp>0.987</erp>
          </limit>
          <provide_feedback>1</provide_feedback>
        </ode>
      </physics>
      <physics>
        <ode>
          <erp>1.5</erp>
          <cfm>10.5</cfm>
        </ode>
      </physics>
    </joint>

Sometimes you want extension tags to be inserted, and sometimes I think you want them to be merged into existing parameter blocks. I think we could improve the behavior here.