Checks how many of the headlines on the site contain links.

        :param obj response: The scrapy response
        :param dict site_dict: The site object from the JSON-File
        :param bool check_self: Check headlines/
                                headlines_containing_link_to_same_domain
                                instead of headline/headline_containing_link

        :return float: ratio headlines/headlines_containing_link

def linked_headlines(self, response, site_dict, check_self=False):
        """
        Checks how many of the headlines on the site contain links.

        :param obj response: The scrapy response
        :param dict site_dict: The site object from the JSON-File
        :param bool check_self: Check headlines/
                                headlines_containing_link_to_same_domain
                                instead of headline/headline_containing_link

        :return float: ratio headlines/headlines_containing_link
        """
        h_all = 0
        h_linked = 0
        domain = UrlExtractor.get_allowed_domain(site_dict["url"], False)

        # This regex checks, if a link containing site_domain as domain
        # is contained in a string.
        site_regex = r"href=[\"'][^\/]*\/\/(?:[^\"']*\.|)%s[\"'\/]" % domain
        for i in range(1, 7):
            for headline in response.xpath('//h%s' % i).extract():
                h_all += 1
                if "href" in headline and (
                            not check_self or re.search(site_regex, headline)
                        is not None):
                    h_linked += 1

        self.log.debug("Linked headlines test: headlines = %s, linked = %s",
                       h_all, h_linked)

        min_headlines = self.cfg_heuristics["min_headlines_for_linked_test"]
        if min_headlines > h_all:
            self.log.debug("Linked headlines test: Not enough headlines "
                           "(%s < %s): Passing!", h_all, min_headlines)
            return True

        return float(h_linked) / float(h_all)