Multiple references to mesh fix

[at669]

Proposed change(s)

URDF Import only creates an asset for a mesh if the asset doesn't already exist (e.g. multiple references to the same mesh), otherwise the asset is loaded.

Useful links (GitHub issues, JIRA tickets, forum threads, etc.)

• AIRO-1117, 1048

• 135

• https://github.com/Unity-Technologies/Unity-Robotics-Hub/issues/285

Types of change(s)

• [x] Bug fix
• [ ] New feature
• [ ] Code refactor
• [ ] Documentation update
• [ ] Other (please describe)

Testing and Verification

Verified that meshes remain between import, play mode, and returning to edit mode, as well as prefabbing and instantiating prefab.

Test Configuration:

• Unity Version: 2020.3.11f
• Unity machine OS + version: MacOS 10.15.7

Checklist

• [x] Ensured this PR is up-to-date with the dev branch
• [x] Created this PR to target the dev branch
• [x] Followed the style guidelines as described in the Contribution Guidelines
• [ ] Added tests that prove my fix is effective or that my feature works
• [x] Increased the test coverage criteria by 3%
• [x] Updated the Changelog and described changes in the Unreleased section
• [x] Updated the documentation as appropriate