WIP: add general notion of "job readiness"
This adds a new generic parameter job_ready that determines whether a given job is ready at a given time in a given schedule. For different workload models (e.g., release jitter, self-suspensions, shared resources, etc.), appropriate definitions of job_ready can be provided. This allows us to define work-conservation and priority-compliance in a general way, which is needed to fully address #12 (closed).
As an example, I've defined job_ready for the workload model with release jitter (and without self-suspensions).
This is just a sketch; what do you think?