USD -> SDF conversion support

[ahcorde]

This issue is meant to track the progress of converting USD to SDF, which is currently a work in progress.

In this PR #736 you can try the converter. To simplify the process of merging things upstream, we should create small PRs

Supported

• Initial PR to set up the structure USD to SDF: Initial commit #827
• Added usd2sdf command line tool USD -> SDF: Added cmd line tool #862
• Physics USD -> SDF: Parse Physics and generated SDF file #863
  • Gravety

In Progress

• Transforms usd -> sdf: Read transforms #871
• World lights USD -> SDF: Added lights attached to the world #875
• Model USD -> SDF: Added model tag #876
• Links USD to SDF: Added links #902
  • mass, inertia, center of mass
• Visual USD to SDF: Added visuals #897
  • Basic shapes
  • Mesh
  • Materials
• Collisions USD to SDF: Added Collisions #898
• Joints USD to SDF: Added Joints #899
  • Revolute
  • prismatic
  • fixed
• Sensors USD to SDF: Added Sensors #900
  • Camera
  • IMU
• World plugins USD to SDF: Added some world plugins #901
• Lights attached to links USD to SDF: Added Lights attached to models #903
• Diff drive plugin USD to SDF: Added diff drive plugin #904

Fixes

• Applied rigid body to Model not to Link USD to SDF: Apply rigid body to Model not to Link #889
• Material fixes USD to SDF: material fixed #895
• Returned errors in USDWorld USD to SDF: Returned errors in USDWorld #896
• Header guard USD to SDF: Fixed header guard #906
• Check valid paths SDF to USD: check and fix if path is valid #976
• Added Material Physics SDF to USD: Added Material Physics #975
• Fixes USD to SDF: some fixes #991

TODO