The two concepts are tightly coupled and it does not make sense
to mix and match them. Therefore, it's cleaner and easier to
understand if both aspects of the preemption model are instantiated
in the same module next to each other.

All "kinds of schedules" and associated validity conditions are being
collected in model.schedule.

By our newly adopted convention, the model/ module should not depend on analysis facts.

This fixes all warnings about ambiguous module names and resolves #49.

It also highlights that we still need to cut down on superfluous
Require Import commands in recently ported files.