Make materials unique

[AlexKaravaev]

I was having compile errors with some models, because util would duplicate some of materials. I can send the model example if needed. I think materials in general will be always unique in .obj, but correct me if I am wrong.

[AlexKaravaev]

mypy still complains about Geometry, but have no idea how this can be related to the change

11:23:51 Alexander.Karavaev@unit-2077 obj2mjcf ±|alex/unique-mtls ✗|→
mypy --install-types obj2mjcf
obj2mjcf/cli.py:94: error: "Geometry" has no attribute "vertices"  [attr-defined]
obj2mjcf/cli.py:94: error: "Geometry" has no attribute "faces"  [attr-defined]
obj2mjcf/cli.py:219: error: "Geometry" has no attribute "geometry"  [attr-defined]
Found 3 errors in 1 file (checked 5 source files)

[kevinzakka]

@AlexKaravaev Probably a mypy upgrade, will fix after merging. In any case, thank you for the PR!