Add [arrives_in] to [sequential_tasks] definition

Note that the prior definition of [sequential_tasks]
did not differentiate between a job coming from the
arrival sequence and any other job. However, all
computable properties (such as [job_respects_task_rtc,
valid_preemption_model, arrivals_have_valid_job_costs,
all_deadlines_of_arrivals_met]) are stated exclusively
for jobs from the arrival sequence. In order to make
the definition of [sequential_tasks] compatible with
computable properties, we add preconditions
[arrives_in arr_seq j1] and [arrives_in arr_seq j2].