Added Add & Clear function to World, Model, and Link

[nkoenig]

Signed-off-by: Nate Koenig [email protected]

🎉 New feature

Summary

Add functions that allow adding SDF elements to World, Model, and Link DOM objects.

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

[mjcarroll]

small typo nit, LGTM

[codecov-commenter]

Codecov Report

Merging #765 (1d67954) into sdf12 (b95f44d) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            sdf12     #765      +/-   ##
==========================================
+ Coverage   89.15%   89.23%   +0.08%     
==========================================
  Files          76       76              
  Lines       12001    12081      +80     
==========================================
+ Hits        10700    10781      +81     
+ Misses       1301     1300       -1     

Impacted Files	Coverage Δ
src/Link.cc	97.16% <100.00%> (+0.50%)	⬆️
src/Model.cc	90.31% <100.00%> (+0.78%)	⬆️
src/World.cc	93.73% <100.00%> (+0.84%)	⬆️

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