Get XML string from classes

The `sdf::Element` class has a convenient `ToString` function to output its contents as an XML string:

https://github.com/osrf/sdformat/blob/e8b30f0bca75c2861b0b21ca042706cc3c117b22/include/sdf/Element.hh#L158-L161

It would be convenient to be able to do the same for the DOM classes such as `sdf::Model`. If these classes have been loaded from an `sdf::Element`, it's possible to call `model.Element()->ToString("")`. But that's not the case if the class was created manually and doesn't have an element pointer.

Start with the ToString declaration in include/sdf/Element.hh and compare it with the DOM classes such as sdf::Model. Determine how manually created classes should produce XML without an element pointer; done means the relevant DOM classes can return their XML representation in both loaded and manually constructed cases.

Written by the indexing model from the issue text.