Hard to set URDF model to static with multiple <gazebo> tags

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/197)) 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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@caguero and I discovered today that if you want to use `` to make a urdf model static, you might run into problems if you have multiple `` tags. For example, the following urdf will not be static after converting to sdf:

~~~

1

~~~

~~~
$ gz sdf -p test/integration/static_test.urdf
Warning [parser_urdf.cc:1115] multiple inconsistent exists due to fixed joint reduction overwriting previous value [true] with [false].

0

~~~

It gives a warning: "Warning [parser_urdf.cc:1115] multiple inconsistent exists due to fixed joint reduction overwriting previous value [true] with [false]."

I think the following code might fix the parsing for this specific model, relying on the fact that sdf models are not static by default, but it may have side-effects for other models.

~~~
diff -r 52b50d2823a67366ccbf17f9976aa36ed7cab4e5 src/parser_urdf.cc
--- a/src/parser_urdf.cc Thu Oct 26 14:18:25 2017 -0700
+++ b/src/parser_urdf.cc Wed Aug 29 17:43:43 2018 -0700
@@ -2346,8 +2346,6 @@
// insert static flag
if ((*ge)->setStaticFlag)
AddKeyValue(_elem, "static", "true");
- else
- AddKeyValue(_elem, "static", "false");

// copy extension containing blobs and without reference
for (std::vector::iterator
~~~

Start in src/parser_urdf.cc around the static flag handling and inspect test/integration/static_test. Run the shown gz sdf -p command to reproduce the conversion and warning. Done means the URDF model remains static after conversion without the inconsistent static warning.

Written by the indexing model from the issue text.