Add support for no gravity link

[AzulRadio]

🎉 New feature

Original issue: #504

A feature I find very useful when I use Gazebo Classic but surprisingly unavailable in Ignition.

I'm targeting Gazebo Fortress because I need this feature for my projects on Fortress. Might need some back/forward porting.

Changes in other repo:
gz-physics: gazebosim/gz-physics#633
sdformat: gazebosim/sdformat#1410

Summary

Add parsing and support for no gravity link when using DartSim engine

Implementation for other physics engines should be easy if they have link no gravity API.

Test it

Spawn the following entity to gazebo.

<?xml version="1.0" ?>
    <sdf version='1.7'>
      <model name='sphere'>
        <link name='link'>
          <gravity>false</gravity>
          <pose>0 0 0.5 0 0 0</pose>
          <visual name='visual'>
            <geometry><sphere><radius>1</radius></sphere></geometry>
          </visual>
          <collision name='collision'>
            <geometry><sphere><radius>1</radius></sphere></geometry>
          </collision>
        </link>
      </model>
</sdf>

The way I do this is to add <gravity>false</gravity> to gz-sim/examples/standalone/entity_creation's inline sdf and run the example.

The black sphere has link gravity disabled and the cube is spawned with GUI and has gravity by default.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[arjo129]

Can you signoff your commits following the instructions here: https://github.com/gazebosim/gz-sim/pull/2398/checks?check_run_id=24713135239

On a side note if you need to achieve the same effect immediately: I achieve similar results using the <static> tag

[AzulRadio]

Can you signoff your commits following the instructions here: https://github.com/gazebosim/gz-sim/pull/2398/checks?check_run_id=24713135239

I signed off with a new GPG key and added the key to Gtihub after I made the PR. So apparently DCO doesn't like that. Now it's fixed.

On a side note if you need to achieve the same effect immediately: I achieve similar results using the <static> tag

Thanks. I'll try that.

[iche033]

@osrf-jenkins run tests please

[iche033]

made some minor changes in 72b8f3c - simplified gravity component data check, and updated style and indentation.

[iche033]

@ahcorde friendly ping to see if you're ok with the new changes and also merging this PR?