Add a dbxref to a feature

        :type feature_id: str
        :param feature_id: Feature UUID

        :type db: str
        :param db: DB Name (e.g. PMID)

        :type accession: str
        :param accession: Accession Value

        :type organism: str
        :param organism: Organism Common Name

        :type sequence: str
        :param sequence: Sequence Name

        This seems to show two attributes being added, but it behaves like those two are one.

        :rtype: dict
        :return: A standard apollo feature dictionary ({"features": [{...}]})

def add_dbxref(self, feature_id, db, accession, organism=None, sequence=None):
        """
        Add a dbxref to a feature

        :type feature_id: str
        :param feature_id: Feature UUID

        :type db: str
        :param db: DB Name (e.g. PMID)

        :type accession: str
        :param accession: Accession Value

        :type organism: str
        :param organism: Organism Common Name

        :type sequence: str
        :param sequence: Sequence Name

        This seems to show two attributes being added, but it behaves like those two are one.

        :rtype: dict
        :return: A standard apollo feature dictionary ({"features": [{...}]})
        """
        data = {
            'features': [
                {
                    'uniquename': feature_id,
                    'dbxrefs': [
                        {
                            'db': db,
                            'accession': accession,
                        }
                    ]
                }
            ]
        }
        data = self._update_data(data, organism, sequence)
        return self.post('addDbxref', data)