Merge branch 'randomizers' into tutorial_sample_project

Get the latest from randomizers into tutorial branch.

com.unity.perception/Runtime/Randomization/Randomizers/SampleRandomizers/Randomizers/BackgroundObjectPlacementRandomizer.cs

         /// </summary>
         protected override void OnIterationStart()
         {
+            if (m_SpawnedObjects == null)
+                m_SpawnedObjects = new List<GameObject>();
+
             for (var i = 0; i < layerCount; i++)
             {
                 var seed = scenario.GenerateIterativeRandomSeed(i);

com.unity.perception/Runtime/Randomization/Randomizers/SampleRandomizers/Randomizers/ForegroundObjectPlacementRandomizer.cs

         /// </summary>
         protected override void OnIterationStart()
         {
+            if (m_SpawnedObjects == null)
+                m_SpawnedObjects = new List<GameObject>();
+
             var seed = scenario.GenerateRandomSeed();
             var placementSamples = PoissonDiskSampling.GenerateSamples(
                 placementArea.x, placementArea.y, separationDistance, seed);