Include about property to any node containing metadata annotation

[cboettig]

Necessary so that extracted RDF nodes still refer to the appropriate point in the DOM. Currently we are not writing this, e.g. https://github.com/ropensci/RNeXML/blob/c04435b8a9759fdd3e21aa2527a7f454b3a166d7/inst/examples/ncbii.xml

Maybe we should just include an about attribute whenever we write an id attribute? Alternatively we have to check if an about attribute is present whenever we add a meta annotation to a node.