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.