OpenMP Parallelization Support for CLASS

Looking at CLASS's OpenMP parallelism, to be able to differentiate through [CLASS](https://github.com/lesgourg/class_public) we need to be able to differentiate the following OpenMP constructs from its C-layer:

- [x] OMP FOR SCHEDULE (STATIC)
- [ ] OMP FOR SCHEDULE (DYNAMIC)
- [x] OMP PARALLEL
- [ ] OMP PARALLEL NUM_THREADS(...)
- [x] OMP PARALLEL FOR
- [ ] OMP PARALLEL FOR SCHEDULE (DYNAMIC)
- [ ] OMP PARALLEL SHARED(...) PRIVATE(...) NUM_THREADS(..)
- [ ] OMP FLUSH(ABORT)

cc @Edenhofer