Possible bug in URDF

[rafaelrojasmiliani]

Hello, I'm not an expert in URDF. I'm trying to deploy Franka on RVIZ with ROS melodic. I'm getting the Franka links spread around using this command

<xacro:panda_arm xyz="0 0 0.5"/>

but when I run

<xacro:panda_arm/>

It is deployed well. I found that the xyz argument is used inside the xacro files in the link/visual/origin and link/collision/origin task as it is done here. I do not know if this is correct, because the xyz argument should move the robot w.r.t. its parent joint and not each one of their links w.r.t. their parent joint. I removed that and it works.

On the other hand if the connected_to arguments is used, a link with that name is declared here. I would imagine that this link is declared outside the arm URDF definition, and this is a re-declaration.

If I'm ok, I can do the pull request.