create ER diagram, refactor to conform to diagram, simplify session management

WorkoutsPlus/Models/Workout.swift

@@ -17,6 +17,8 @@ final class Workout: Nameable, Hashable {
 		
 		//		The name of my workout is: Recommended Routine, My Marathon Workout
 		@Attribute(.unique) var name: String
+		var defaultRestTime: TimeInterval = 60
+		var useDefaultRestTime: Bool = false
 		
 		// Icon
 		var workoutIconSystemName = "figure.run"
@@ -33,6 +35,10 @@ final class Workout: Nameable, Hashable {
 				self.name = name
 		}
 		
+		func start() -> WorkoutSession {
+				return WorkoutSession(start: self)
+		}
+		
 		func add(workoutItem: WorkoutItem) {
 				self.workoutItems.append(workoutItem)
 				updateWorkoutItemsPositions()