Some bugs in using SDF extensions from URDF

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/232)) by Steve Peters (Bitbucket: [Steven Peters](https://bitbucket.org/%7B2ccfed09-18b8-4921-8d58-15ef01092802%7D/), GitHub: [scpeters](https://github.com/scpeters)).**

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The [urdf_gazebo_extensions.cc](https://github.com/osrf/sdformat/blob/master/test/integration/urdf_gazebo_extensions.cc) test uses [urdf_gazebo_extensions.urdf](https://github.com/osrf/sdformat/blob/master/test/integration/urdf_gazebo_extensions.urdf) to demonstrate the functionality for inserting SDF extension tags into URDF files with the `` tag, as described in the description of [pull request #225](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/225). There are some issues with this functionality that I noticed while reviewing [pr #506](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/506), and I'm working on illustrating them with a test in branch [urdf_gazebo_extensions_fix](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/branch/urdf_gazebo_extensions_fix).

A summary of the changes so far in that branch:

* 36b9c67002bdc5ec4f11781942b09faed5245861: remove `joint23` since it creates a kinematic loop and makes the URDF invalid
* 1c19e26b490eba522744ed7729aee334b4e31098: fix some expectations for joint12 since it is looking for elements in the wrong place
* e07490a657b42a738a4f6aca07ce6eb69429949b: add `//joint/physics/ode/cfm` and `//joint/physics/ode/erp` tags using the syntax described in [pr #225](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/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:

~~~

0 0 -1 0 -0 1.5708
link0
link1

1 0 0

-1e+16
1e+16


1.1111
2.2222
0.234
0.567



1

0
0
0.56789

123
0.987

1




1.5
10.5



~~~

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.