trimesh load error

Hi. Thanks for the great package for parsing the urdf files. 

I notice there's a small error in utils.py. Is [line 225](https://github.com/mmatl/urdfpy/blob/5466842899b33bd549e8f9e2a9a987bd5e37373b/urdfpy/utils.py#L225) of utils.py intended to load the mesh from a file path? Then I guess `trimesh.load_path(filename)` will be the best option.