parser_urdf: add //frame for reduced links/joints (garden)

[scpeters]

Draft pull request until #1137 and #1126 are merged to sdf13

🎉 New feature

Forward port of #1148, built on top of #1137 and #1126, related to #1110

Summary

This differs from #1148 by using sdf::Frame::ToElement()::ToString to serialize the new frames to an XML string and also has better floating point precision due to #1137.

Original summary from #1148

Currently the location of some links and joints is lost when fixed joints are reduced and links lumped together. For example, the first snippet below is a URDF with two links outlined in red and a fixed joint outlined in blue. The child link has a visual outlined in green. The second snippet shows what results when converting to SDFormat, with the fixed joint and child link removed, and the visual moved to the parent link.

This process of "fixed joint reduction" reduces the complexity of a simulation, particularly for solvers using maximal coordinates, but it does lose some frame information. This pull request adds //model/frame tags with the same name and pose of the reduced links and joints. For example, the following would be added for the examples shown above.

    <frame name='fixed_joint' attached_to='base'>
      <pose>0 0 0 0 0 0</pose>
    </frame>
    <frame name='child_link' attached_to='fixed_joint'>
      <pose>0 0 0 0 0 0</pose>
    </frame>

Test it

Compile and run INTEGRATION_urdf_gazebo_extensions

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.

[codecov]

Codecov Report

Merging #1149 (7352b5a) into sdf13 (dc5720a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            sdf13    #1149      +/-   ##
==========================================
+ Coverage   87.00%   87.02%   +0.01%     
==========================================
  Files         125      125              
  Lines       15834    15852      +18     
==========================================
+ Hits        13777    13795      +18     
  Misses       2057     2057              

Impacted Files	Coverage Δ
include/sdf/Param.hh	80.72% <100.00%> (ø)
src/parser_urdf.cc	87.91% <100.00%> (+0.13%)	⬆️

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[scpeters]

these changes are now in #1182