Require removal and model cleanup, part 1

added 28 commits

• a258c252 - model reorg: move priorities definition to own folder
• 73653440 - model reorg: move EDF priority definition to model.priority
• ba758466 - model reorg: have only one notion of priority-driven schedules
• 4a80f261 - model reorg: move TDMA facts to analysis
• 393d2007 - model reorg: move preemption-time notion to model.schedule
• 9933d50d - model reorg: job.v is an analysis def; job_deadline a task property
• 65f28df9 - fix EDF priority definition to not depend on task abstraction
• eca90fdf - model reorg: 'sequential tasks' is a task property
• 42e6a42b - model reorg: move task concept to model.task.concept
• 9c7b4f30 - model reorg: move task preemption parameters to model.task.preemption
• 25754fea - model reorg: move task preemption models to model.task.preemption
• 572c699b - model reorg: move preemption.valid_schedule -> schedule.limited_preemptive
• 4e9768b1 - model reorg: task.sequential -> task.sequentiality
• 540acffd - model reorg: fold RTCT validity into task.preemption.parameters
• 7b64c4b0 - model reorg: fold RTCT instances into task.preemption.*
• f96feefc - model reorg: get rid of purely re-export models
• ba086071 - preemption_time.v should not depend on task concept
• 7b38326f - split job and tasks aspects in preemption.valid_model
• 4d5c9497 - model reorg: merge job_preemptable def and validity
• 1306c75c - model reorg: flatten model.preemption
• 12b573c3 - remove task dependency in job limited-preemptive preemption model
• 0bc2ea0b - remove unneeded Require in model.task.preemption.parameters
• 390228e8 - remove unneeded 'Require' statement in analysis spec
• ba8dc43b - Don't Export modules that introduce new Instances
• d14a37c0 - Improve comments in analysis
• 0089c2ce - Update README
• 1b7dd375 - remove unnecessary Require statements in model.task.preemption
• ddcd2790 - update guidelines to state `Require Import|Export` rules

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merged