Should detect if OBJ is composite and issue a warning

[azeey]

Mujuco doesn't textures in a composite OBJ file (multiple submeshes in one OBJ files). It would be nice to detect if an OBJ file is composite and issue a warning along the lines of "extures ignored in mesh "mesh.obj" since it is composite, in order to load a composite mesh in MuJoCo please use github.com/kevinzakka/obj2mjcf"

The tool linked there can convert composite OBJ files to simple OBJs with individual textures extracted from the composite OBJ file.