Ingest xml to create a citation

        :param resource: XML on which to do xpath
        :param element: Element where the citation should be stored
        :param xpath: XPath to use to retrieve citation

        :return: XmlCtsCitation

def ingest(cls, resource, element=None, xpath="ti:citation"):
        """ Ingest xml to create a citation

        :param resource: XML on which to do xpath
        :param element: Element where the citation should be stored
        :param xpath: XPath to use to retrieve citation

        :return: XmlCtsCitation
        """
        # Reuse of of find citation
        results = resource.xpath(xpath, namespaces=XPATH_NAMESPACES)
        if len(results) > 0:
            citation = cls(
                name=results[0].get("label"),
                xpath=results[0].get("xpath"),
                scope=results[0].get("scope")
            )

            if isinstance(element, cls):
                element.child = citation
                cls.ingest(
                    resource=results[0],
                    element=element.child
                )
            else:
                element = citation
                cls.ingest(
                    resource=results[0],
                    element=element
                )

            return citation

        return None