[export] Ensure that filenames are prefixed by file:// or package://

[arntanguy]

This PR ensures that filenames in the urdf/yaml obtained by to_urdf/to_yaml are always prefixed by either file:// or package://. This is a requirement for a lot of ROS tools (such as rviz).

For example the following cannot be displayed in rviz

<mesh filename="/usr/local/share/panda_prosthesis/meshes/support_femur.stl" scale="0.001 0.001 0.001"/>

but with the file:// prefix everything is fine.

<mesh filename="file:///usr/local/share/panda_prosthesis/meshes/support_femur.stl" scale="0.001 0.001 0.001"/>

Note that this assumes that in case no valid prefix is provided that the intent was to load a local file.

[gergondet]

Looks ok.

You need to update the reference URDF used in the tests: https://github.com/jrl-umi3218/RBDyn/blob/master/tests/ParsersTestUtils.h#L220