modify definitions of priority policy compliance

The current definitions of `respects_JLFP_policy_at_preemption_time`
and `respects_FP_policy_at_preemption_time` use the predicate
`hep_job_at`. This predicate does not make sense for `JLFP`
and `FP` policies. To avoid this, these definitions should
instead use the definition `respects_JLDP_policy_at_preemption_point`.
Doing this requires coercions from `JLFP` to `JLDP` and from
`FP` to `JLFP`. This patch also introduces these coercions.