Evaluate support of SDF → URDF conversion

[diegoferigo]

Currently rod supports URDF indirectly by performing a under-the-hood conversion to SDF, assuming that Gazebo Sim and sdformat are found in the system.

However, some of our libraries (e.g. iDynTree) do not yet support SDF, and this is the reason why most of our code still relies on URDF files. While waiting that SDF support propagates, it might be a good idea to implement a SDF → URDF conversion.

The URDF structure is much simpler than SDF, and converting rod.* objects would be pretty easy. Of course, there would be some information getting lost.

Extra care would be however necessary in the following cases:

• Frame semantics logic since URDFs cannot specify the reference frame of pose elements
• Frames, that could be optionally converted to (fixed_joint → empty_link) chains
• Extra elements that could be defined in custom <gazebo> tags

[diegoferigo]

This should be quite simple with the FrameConvention.Urdf introduced in https://github.com/ami-iit/rod/pull/16.