Move the URDFs to a git submodule

[jbmouret]

We have controllers that require URDFs... but we do not want to install robot_dart on the robot.
=> I think we should have a git submodule with the URDF so that we can have a URDF library outside of robot_dart

The main downside is that a deep clone will be required (and that @costashatz does not like git submodules). Maybe we could use git subtrees? https://www.atlassian.com/git/tutorials/git-subtree

[costashatz]

I guess #166 solve this no? @jbmouret ?

[jbmouret]

yes