add load ans save methods to ColoredTrimesh

[martinResearch]

add load and save methods to and from ColoredTrimesh potentially using the trimesh lib as backend.
when saving textured mesh

• make sure we do not duplicate edges that have different uv in the different faces (which might be the case if we use trimesh)
• add option to provide a texture name that default to the name of the obj file with mtl and png extension to avoid clashes on the material files (unlike trimesh's export method that defaults to material0.png)
• provide option to choose the image file format (jpg or png)