/// Recommended way to create systems in general: pool.CreateSystem(new MySystem());
 public ReactiveSystem(Pool <T> pool, IMultiReactiveSystem <T> subSystem) :
     this(subSystem, CreateGroupObserver(pool, subSystem.Triggers))
 {
 }
 /// Recommended way to create systems in general: pool.CreateSystem(new MySystem());
 public ReactiveSystem(Pool <T> pool, IReactiveSystem <T> subSystem) :
     this(subSystem, CreateGroupObserver(pool, new[] { subSystem.Trigger }))
 {
 }