Allow physics presets to be <include>'ed

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/87)) 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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Physics presets were added in [pull request #146](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/pull-requests/146) by @jacquelinekay and it would be very cool if users could create libraries of different physics presets that could be ``'ed into world files.

For example, currently models can be inserted with:
~~~

cup_1
model://plastic_cup
-0.8 250 1.015 0 0 0

~~~

If there was a an sdf file named `my_physics.sdf` like the following:
~~~

0.000000 0.000000 -9.810000
0.001000
1000


quick
100
0
1.000000




sequential_impulse
100
1.000000



0.001
0.01

~~~
and if it was in the model database, perhaps it could be included like:
~~~

model://my_physics

~~~

@rosebudflyaway