URL of document on ProvStore

        :Example:
          >>> stored_document.url
          'https://provenance.ecs.soton.ac.uk/store/documents/148'

def url(self):
        """
        URL of document on ProvStore

        :Example:
          >>> stored_document.url
          'https://provenance.ecs.soton.ac.uk/store/documents/148'
        """
        if not self.abstract:
            return "%s/documents/%i" % ("/".join(self._api.base_url.split("/")[:-2]), self.id)