Trigger model on n new occurrences

The threshold "n" is a nullable field, set for each disease group: model_run_min_new_occurrences on disease_group table. The model should NOT run if its value is null, as this indicates a new disease group that has not been verified by an admin.

The model SHOULD run if:
- lastModelRunPrepDate is null, because it has never been run before
- OR a week has elapsed since the last model run prep date
- OR there have been more than "n" new occurrences