Time constraints corresponding to the different AK categories are introduced.
They are added to AKs belonging to the category and fulfilled by timeslots that are contained in a default slot of this category.
To that end, this PR implements no. 3 in the "merging" strategy of #10, i.e. we check if we would have to merge partially overlapping timeslots and error out if this is the case. This places the burden to design appropriate schedules on the conference organizers.
