[ign] paths in worlds/models need to be absolute

[kruusamae]

No description provided.

[FabianPG11]

The problem is that SDF file doesn't support substitution as we used to do with xacro $(find my_package)
Ignition gazebo can find a model as long as its parent path is listed on the environment variable:
e.g. export IGN_GAZEBO_RESOURCE_PATH=~/robotont_ws/src/robotont_gazebo/ign_worlds

Inside of SDF files I have to put the entire path to be able to load a resource like:
file://src/robotont_description/meshes/body.stl (but this means the world/model has to be launched from the root path)