Make URDF configurable for instances of inidivdual robots (instantiate a generic URDF)

[marc-hanheide]

We need a general approach how we can account for changes in the poses of joints in the URDF for individual robot.

While the structure of each robot platform is the same and generated from the urdf/xacro setup in hunter_pltf_description/description, there are small idiosyncratic differences for each robot, e.g. around tolerances for mount points of sensors, antennas, etc.

We need a generalisable approach to inject the instance-specific information into the URDF tree, while trying to fix the structure of the platform and maximising re-use. So we don't want individual xacro files for each instance of a robot, but rather use either environment variables or small robot-specific yaml files (one ofr each specific robot instance).