DeliveryPart: Add Unloading type instead of IsLesewagen

Elwig/Resources/Sql/34-35.sql

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+-- schema version 34 to 33
+
+ALTER TABLE delivery_part ADD COLUMN unloading TEXT DEFAULT NULL;
+UPDATE delivery_part SET unloading = 'pumped' WHERE lesewagen;
+UPDATE delivery_part SET unloading = 'box' WHERE (SELECT zwstid IN ('H','S') FROM delivery d WHERE (d.year, d.did) = (delivery_part.year, delivery_part.did));
+ALTER TABLE delivery_part DROP COLUMN lesewagen;