Exporting to URDF - Githubissues

This PR implements the SDF -> URDF conversion by exporting the in-memory model to URDF. Closes #6.

Links and joints are converted directly to URDF exploiting the already existing FrameConvention.Urdf that expresses the poses of a model compatibly with URDF^1
Frames are converted to (fixed_joint->fake_link) pairs and attached to the first suitable frame parent
Gazebo elements are included for preventing lumping frames in case the exported URDF is converted back to SDF[^2]
The URDF exporter accepts a list of fixed joints to keep for the same reason, and the proper Gazebo elements are included in the resulting URDF
Not all SDF features can be converted to URDF, e.g. links must have a trivial pose w.r.t. their parent joint
The root of the URDF model is the canonical link, therefore the `model frame of SDF is ignored

[^2]: this might be removed in the future if sdformat start recognizing them as frames when an URDF is loaded

ami-iit / rod