Allow physics presets to be <include>'ed

[osrf-migration]

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

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Physics presets were added in pull request #146 by @jacquelinekay and it would be very cool if users could create libraries of different physics presets that could be <include>'ed into world files.

For example, currently models can be inserted with:

<include>
  <name>cup_1</name>
  <uri>model://plastic_cup</uri>
  <pose>-0.8 250 1.015 0 0 0</pose>
</include>

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

<?xml version="1.0" ?>
<sdf version="1.5">
  <physics type="ode" name="my_physics">
    <gravity>0.000000 0.000000 -9.810000</gravity>
    <max_step_size>0.001000</max_step_size>
    <real_time_update_rate>1000</real_time_update_rate>
    <ode>
      <solver>
        <type>quick</type>
        <iters>100</iters>
        <precon_iters>0</precon_iters>
        <sor>1.000000</sor>
      </solver>
    </ode>
    <bullet>
      <solver>
        <type>sequential_impulse</type>
        <iters>100</iters>
        <sor>1.000000</sor>
      </solver>
    </bullet>
    <simbody>
      <accuracy>0.001</accuracy>
      <max_transient_velocity>0.01</max_transient_velocity>
    </simbody>
  </physics>
</sdf>

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

<include>
  <uri>model://my_physics</uri>
</include>

@rosebudflyaway

[osrf-migration]

Original comment by Jackie K (Bitbucket: jacquelinekay).

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I agree, with the small caveat that the include statement should look like

<include>
  <uri>physics://my_physics</uri>
</include>

so as not to conflate physics profiles with models.