Fields set at import from solver result json

The solver result consists of assignments (cf. Input/Output spec of the KoMa tool):

{
  "ak_id": "UNIQUE ID OF THE AK",
  "room_id": "UNIQUE ID OF A ROOM",
  "timeslot_ids": ["time_slot_id"],
  "participant_ids": ["participant_id"],
}

The import is realized in Event::schedule_from_json. For each scheduled AK the following fields are set in the import:

• the room is set to the room the AK is scheduled to take place in
• the start is set to the start of the first timeslot the AK occupies
• the duration is set to the duration of all timeslots that the AK occupies

Setting room and start is inevitable. But I think we can skip setting the duration. It is a required field for the JSON export to the solver (cf. AKSlot::as_json) so it must be set in the first place. And setting it like this disallows the AKs to have durations outside of the grid for the solver.