Setup layer structure

[ft-lab]

Description

Fixes #16

Here we implemented the same layer structure as mujoco-usd-converter.

• The body.py, material.py, mesh.py, and scene.py files are not yet implemented.
  These will be implemented in separate issues.
• tests/testCli.py: The test_run method was missing the detection of urdf files with errors, so this has been fixed.
• tests/testCli.py: Added unit tests for test_no_layer_structure, test_no_physics_scene, and test_comment.
• Added simple-primitives.urdf file to "tests/data".
  This is an example of what needs to be replaced as usd prim names.
• Coverage Rate: _flatten.py has low coverage, this should be resolved when we implement texture conversion for materials.

Checklist

• I am familiar with the Contributing Guidelines.