Should have a parser option or command line flag to preserve includes when converting to string

[azeey]

Desired behavior

It would be nice for information contained in the <include> element be preserved when converting sdf::Element objects to string. This information is being stored in sdf::Element as of #509. Currently, the included models (or other entities) are expanded into the parent model with the parameters in the <include> element being used to override values in the included model. Thus, when the corresponding sdf::Element of the parent model is converted to a string, we get the string of the expanded child model.

Alternatives considered

The alternative is to always preserve the <include> element, but this would be a breaking change.

Implementation suggestion

Add an option in sdf::ParserConfig and and a command line flag to ign sdf -p

[jennuine]

Per VC, will leave ign sdf -p the way it is currently (expands <include> tags) and will add new command flag to preserve <include>