The BSD version of `find` needs to be given '.' as the search path.

- Add definitions related to APA scheduling
- Prove correctness of reduction-based RTA for APA scheduling (FP and EDF)
- Add implementation of a weak APA scheduler
- Update definition of taskset to assume uniqueness
- Modify names and comments to improve readability
- Remove strong assumptions about priority order in FP scheduling
- Add tests with FP RTA for every model
- Add tests for RTA with parallel jobs

We use simpler, more pessimistic interference bounds to
prove that Bertogna and Cirinei's RTA works for parallel jobs.