Retrieve citedby documents from a given PID number.

        pid: SciELO PID number
        metaonly: will retrieve only the metadata of the requested article citations including the number of citations it has received.
        from_heap: will retrieve the number of citations from a preproduced report, it will not fetch the api. Much faster results but not extremelly updated.

def citedby_pid(self, pid, metaonly=False, from_heap=True):
        """
        Retrieve citedby documents from a given PID number.

        pid: SciELO PID number
        metaonly: will retrieve only the metadata of the requested article citations including the number of citations it has received.
        from_heap: will retrieve the number of citations from a preproduced report, it will not fetch the api. Much faster results but not extremelly updated.
        """

        if from_heap is True:
            result = citations.raw_data(pid)

            if result and 'cited_by' in result and metaonly is True:
                del(result['cited_by'])
                return result

            if result:
                return result

        result = self.client.citedby_pid(pid, metaonly=metaonly)

        try:
            return json.loads(result)
        except:
            return None