This returns the system identifier (URI) of the entity in the
source document that contains the context node. There are no arguments.
Note: the system identifier will often be the same as the base URI. It may also be the same
as the document URI. However, there are cases where all three may be different. In the case of a document
constructed from a JAXP Source, the system identifier is the value supplied in the setSystemId() method.
In other cases it will generally be the same as the base URI of the document node at the root of the tree.