Particle emitter based on SDF

[nkoenig]

🎉 New feature

Summary

Creates new system for particle emitters that utilizes the SDF particle emitter.

A couple notes:

1. The original ParticleEmitter system is kept in order to ensure backward compatibility. The ParticleEmitter will be deprecated in Ignition Fortress, and removed in Ignition G. The ParticleEmitter2 system will be deprecated in Ignition G and copied to ParticleEmitter. Finally, ParticleEmitter2 will be removed in Ignition H.
2. The 'link.protomessage will be updated to include arepeated ParticleEmitterfield in Ingition Fortress, and theparticle_emitter.protomessage will be updated to have atopicfield. These two changes will let me remove the service inParticleEmitter2that returns the list of existing particle emitters and the inclusion of topic information in theparticle_emitter.proto` header.

This is part of a series of pull requests designed to support particle emitter in gz3d.
See also:

• Particle emitter sdformat#528
• Added particle_emitter vector message gz-msgs#149
• Support particle_emitters in the websocket server gz-launch#104

Test it

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

Note to maintainers: Remember to use Squash-Merge

[codecov]

Codecov Report

Merging #730 (d70a32b) into ign-gazebo4 (8829840) will decrease coverage by 0.05%.
The diff coverage is 63.78%.

❗ Current head d70a32b differs from pull request most recent head 611c85a. Consider uploading reports for the commit 611c85a to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo4     #730      +/-   ##
===============================================
- Coverage        65.51%   65.45%   -0.06%     
===============================================
  Files              237      238       +1     
  Lines            17249    17434     +185     
===============================================
+ Hits             11300    11412     +112     
- Misses            5949     6022      +73     

Impacted Files	Coverage Δ
include/ignition/gazebo/SdfEntityCreator.hh	100.00% <ø> (ø)
src/systems/particle_emitter2/ParticleEmitter2.cc	37.66% <37.66%> (ø)
src/Conversions.cc	81.56% <77.64%> (-0.47%)	⬇️
src/SdfEntityCreator.cc	93.54% <100.00%> (+0.28%)	⬆️
src/Util.cc	95.72% <100.00%> (+0.24%)	⬆️
src/SimulationRunner.cc	92.95% <0.00%> (-1.06%)	⬇️

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[iche033]

works for me. Just one minor comment left.