Agreement with the data organization

Robot folders hasn't exactly organization as original ROS packages. This point was raised here: https://gepgitlab.laas.fr/loco-3d/crocoddyl/merge_requests/107. And I copy from there the following motivations of actual description:

• A package.xml is used by ROS, we probably don't want to add ROS dependency.
• We don't depend on xacro files, so we don't need to create URDF folder for putting just one file.

Additionally we might want add extra information for ensuring that users don't use these models for robot deployment.

@ostasse, @gsaurel, @jmirabel and @nmansard let's find an agreement here.