SDF -> USD conversion support

[adlarkin]

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

@ahcorde if there's anything I'm missing in the feature list below (or if anything seems incorrect/out of date), feel free to modify the feature list directly.

Supported

• World's gravity information
• Lights
  • //light/direction is currently ignored (USD lights will point down by default)
  • //light/diffuse and //light/specular are currently ignored, but a potential workaround for these elements is to apply a material to the USD light

In Progress

• Links (sdf -> usd Parse links and ground planes #828)
  • static/non-static
  • mass, inertia, center of mass
  • basic ground plane support (USD doesn’t have a plane geometry yet, so the workaround is to make a wide, thin box that represents a place)
  • lights attached to links
  • visual
    • Box, cylinder, sphere, mesh, capsule, plane, ellipsoid geometries
      • Mesh geometry support is limited to points, lines, and triangles submesh primitive types
  • collisions
    • Limited support. Currently, a collision is created for every visual, with the collision geometry matching the link’s visual geometry. SDF supports setting collisions that can differ from link geometries (perhaps users want a simpler collision shape, for example)
• Sensors (sdf -> usd: Added sensor #830)
  • Camera, lidar, imu are supported
    • //lidar/scan/horizontal/samples and //lidar/scan/vertical/samples currently aren’t being parsed to the USD lidar sensor
• Materials (sdf -> usd: Materials #831)
  • Ignition fuel resources are searched for ($HOME/.ignition/fuel)
• Joints (sdf -> usd converter: add joints #837)
  • Support for revolute, ball, fixed, and prismatic joint types
• USD-specific error codes (Add USDError class #836)

Not Supported

• SDF files with multiple worlds (the sdf2usd parser executable only supports SDF files with exactly one world)
• Nested models
• Plugins

Additional context

Here are past PRs related to this work that have been merged:

• make SDF to USD a separate component of sdformat #817
• Add sdf -> usd light functionality to usd component #818

[ahcorde]

As I commented in this PR #762 This task is done!