[parsers] Load material from URDF

[gergondet]

This loads the material tag information from URDF files

@BenjaminNavarro do you support these tags in the yaml format? If so I will add it to the YAML parser as well

[BenjaminNavarro]

No we don't use the tag, at least right now. But it might still be a good idea to implement it in order to keep feature parity between the two parsers and keep the URDF<->YAML converter functional

[gergondet]

Thanks for the feedback.

I've added materials parsing to the YAML format and material generation for the to_urdf and to_yaml functions.

I'll merge this and release a new version of RBDyn