AKs with multiple slots in JSON export

One AK object may have multiple AKSlots that need to be scheduled. The export to JSON code should generally work for AKs with multiple occurrences, as should Event::schedule_from_json.

As I understand it, the slots have no ordering information to consider. So we simply need to make sure to not schedule them simultaneously.

I think the following tasks are open:

• Add constraints to disallow the occurrences of the AK to be scheduled in an overlapping fashion. We can simply reuse a solution for #4 (closed).
• The only information in the JSON export for the solver on the represented AK is the AK name. We might want to consider to add the ID of the AK to the info dict
• Check the preference polling tool that it works well with multiple AK occurrences