create a PTBDocumentGraph from a string containing PTB parses.

def fromstring(cls, ptb_string, namespace='ptb', precedence=False,
                  ignore_traces=True):
        """create a PTBDocumentGraph from a string containing PTB parses."""
        temp = tempfile.NamedTemporaryFile(delete=False)
        temp.write(ptb_string)
        temp.close()
        ptb_docgraph = cls(ptb_filepath=temp.name, namespace=namespace,
                           precedence=precedence, ignore_traces=ignore_traces)
        os.unlink(temp.name)
        return ptb_docgraph