Migrate from simulation/interaction/collaboration to activity/groupActivity

We will deprecate the entry points `simulationOf`, `interactionOf`, `collaborationOf`, and `unsafeCollaborationOf`, and replace them with new entry points `activityOf`, `groupActivityOf`, and `unsafeGroupActivityOf`.

The new entry points differ in that they do not take a separate time-step function as an argument. Instead, the time step function is included in `Event` as a new constructor called `TimePassing`.