Is property_path sound?

[bkircher]

property_path, indexed_property_path, (and extended_property_path) are participants in a hierarchy. Why?

Also, we pass property_path by value (as a copy) in a lot of different places in the API, coercing indexed_property_path eventually.

Finally, property_path::field_uri is used in different places, with <FieldURI="" /> hand-coded. Other types in the API provide a to_xml member-function. Consider adding to_xml to property_path. You could circumvent the type coercion problem by using a closure (std::function) internally.

[bkircher]

This is addressed in PR #38