Should store an included model's source file name

[azeey]

This can be useful when a world is saved. Relates to a TODO in https://github.com/osrf/sdformat/blob/144f82cf8b994abd2a566a7bbe6ae873a9d92403/src/parser.cc#L1075-L1082

[EricCousineau-TRI]

Can you illustrate this workflow, or point to a Gazebo tutorial / unittest that illustrates the nested model filenames being kept?

[azeey]

I don't think there is currently an example of this, but when saving worlds in ign-gazebo, there is an option to preserve the <include> tag as opposed to expanding it to a <model>. This worked by making use of sdf::Element::FilePath of the included model (https://github.com/ignitionrobotics/ign-gazebo/blob/5348afa4b5ae4349f72aca51af01db856ae65b90/src/SdfGenerator.cc#L101-L124). This was somewhat of a workaround for the lack of information regarding what file the model was loaded from. It is also unable to retrieve any other information included in the original <include> element, such as <plugin> or any custom elements.

#509 addresses this by storing the original content of the <include> element so it can be retrieved at a later time for such purposes.

[EricCousineau-TRI]

Sweet! Thanks!