New example: get an ECM snapshot from an external program

[chapulina]

🎉 New feature

Summary

Add an example program showing how to instantiate a copy of the ECM from another process. This makes use of the serialized state message, which can be acquired from the /state service or topic. This makes it convenient to access all entity and component data without the need for dedicated messages for each component type.

Test it

See the new README for instructions.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

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[codecov]

Codecov Report

Merging #859 (8b47a82) into ign-gazebo3 (5a3b1b8) will not change coverage.
The diff coverage is n/a.

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##           ign-gazebo3     #859   +/-   ##
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  Coverage        77.98%   77.98%           
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  Files              216      216           
  Lines            12131    12131           
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  Hits              9460     9460           
  Misses            2671     2671           

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[ahcorde]

Is there any way to do the opposite? Set the ECM using a external program ?

[chapulina]

Is there any way to do the opposite? Set the ECM using a external program ?

Interesting idea, we don't have a transport interface for that right now, but it could be added. It would be quite powerful though, so I think that would be geared towards advanced users.