Fix CSV rendering for polymorphic types

Up to now, a query restriction for `fields` to a GET request was ignored
for polymorphic types. For a GET Request requesting CSV the query
restriction for `fields` must be honored, though!
Add `ranking_list_points_lp` to Boat_in_Regatta

Plus `ranking_list_factor` and `starters_rl` to `SRM.Regatta`
(needed by `SRM.Boat_in_Regatta.ranking_list_points_lp.computed`).
Add support for true and floor division to SAW.QX