Issue with urdf file path

[mhyatt000]

in my experience:
urchin expects the path_to_mesh_assets needs to be local to __file__

but it ought to be local to the urdf_path itself

[mhyatt000]

ie URDF.load(...file.urdf) fails if it references stl that is not located directly where python is located.

- main.py
- path/to/file.urdf
  - path/to/assets/link1.stl