MaterialProp not exported into the simmechanics file

[diegoferigo]

I'm digging into the python code for adding colors in the generated urdfs. The models at the current stage are all white, and despite the urdf --> sdf conversion for material elements still doesn't work, I realized that in the first place the MaterialProp element of the simmechanics file is never specified. This would mean that 1) it is not exported from the CAD, or 2) It is not handled properly from Simscape Multibody Link.

The following message is displayed:

Material has neither a color nor texture.

Side note: these errors are suppressed during the generation process and imho they should't be.

Check the Heidelberg model as reference.

cc @traversaro @fiorisi