Allow parallel scheduling of multiple AKs in `proxy` rooms

We might want to model AKs where the "room" is only meeting point via a "proxy room" with unlimited capacity. However, in contrast to general seminar rooms multiple AKs might meet at the same time at a meeting point.

To allow this, I see two options:

1. We could adapt the LP constraints for this case
2. We could introduce multiple "instances" of a proxy room (e.g. meeting-point-1, ..., meeting-point-N that all represent the same meeting point). This would allow us to schedule multiple AKs in the same room in parallel (in the example N AKs). However, in this case we should catch this case at the import phase and recombine all instances of a proxy room into one.