A vector of all possible ballot types.
See also
Other FOKUS metadata:
all_argument_sides,
all_ballot_dates,
all_cantons,
all_election_seat_types,
all_langs,
all_lvls,
all_main_motive_types,
all_name_types,
all_postal_dispatch_types,
all_postal_dispatch_ways,
all_prcds,
all_proposal_types,
all_qstn_groups,
all_response_option_types,
qstnr_item_keys