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 URDF¹
• 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 SDF²
• 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