Detachable joint: support for nested models of the same name

[arjo129]

🦟 Bug fix

Fixes #<NUMBER>

Summary

Not sure if this is a bugfix or a feature tbh. Currently, if we are loading multiple models with child models that have the same name, the detachable joint will pick the wrong one. This is because given the current logic we only check for a child model by name. My understanding is that we wrote this plugin so we can detach sibling models.

The proposed fix in this PR is that if there are multiple models with a similar name, the system will look for the model which is a child of the parent model. This is by no means the best solution, as I do not handle grandchildren or the case where there are multiple children of the same name but it is a solution. Would be happy to have other suggestions as to how I should handle this.

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [x] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
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[codecov[bot]]

Codecov Report

Merging #1097 (a4cd125) into ign-gazebo6 (97f2e9c) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           ign-gazebo6    #1097      +/-   ##
===============================================
+ Coverage        61.88%   61.89%   +0.01%     
===============================================
  Files              274      274              
  Lines            22441    22447       +6     
===============================================
+ Hits             13887    13893       +6     
  Misses            8554     8554              

Impacted Files	Coverage Δ
src/systems/detachable_joint/DetachableJoint.cc	72.50% <100.00%> (+2.22%)	:arrow_up:

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[azeey]

Removing beta label. Let me know if you're planning to get this in before Harmonic is released.

[arjo129]

I'll address this after the code freeze.