Parse meshes specified via package:// URIs

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URDF models used in the ROS world typically contains meshes that are specified via package:// URIs, also as that is the only way to specify meshes without absolute filenames, at least according to the URDF specification.

While many libraries (including urchin) also support an undocumented URDF extensions in which meshes location specified by filename="this/is/a/path/to/a/mesh.stl" are interpreted as relative paths, "official" URDF files must use meshes specified via package.xml.

To support meshes specified via package:// URI in urchin without the need of depending on ROS libraries not available in pypi, I think we could add support for resolving package:// URIs via the https://github.com/ami-iit/resolve-robotics-uri-py, that is a no-dependency pure python library that can resolve package:// URIs, just using environment variables (like ROS_PACKAGE_PATH or AMENT_PREFIX_PATH) that ROS or most other URDF-capable libraries already define and support, see for example:

• Pinocchio: https://github.com/stack-of-tasks/pinocchio/blob/17c479e2fe91905122713643f489feb8ddf135a3/src/parsers/urdf/geometry.cpp#L470, https://github.com/stack-of-tasks/pinocchio/blob/9389400a6018e97ec3822796d465a4b801630a1f/src/utils/file-explorer.cpp#L45
• idyntree: Load meshes that are specified using the package:// URDF URI  robotology/idyntree#291
• raisim: OgreVis::loadMeshFile: Support URDFs with package:// paths leggedrobotics/raisimOgre#25
• DART: https://github.com/dartsim/dart/blob/46abc668dbbcdc58fb63e07f7d8ffb15e834da0a/dart/utils/PackageResourceRetriever.hpp
• Drake: https://github.com/RobotLocomotion/drake/blob/4e5b1133d89c8499ed3aed0fb3905672b9bdf175/multibody/parsing/package_map.cc

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fyi @LudovicaDanovaro