keep metadata statements relative to .nt file

the output files contain some statements about themselves, but as the statements are serialized in n-triples for performance reasons, they need to be absolute (no base= argument to the serializer), and thus get unfindable when the file is moved or accessed via a different protocol.

possible solutions include serving the data on a fixed online location or moving to a different serialization format (possibly less efficient with an even more efficient side database like pickled graphs).