stack observations

Project/Assets/ML-Agents/Examples/Reacher/Prefabs/Agent.prefab

   m_EditorClassIdentifier: 
   m_BrainParameters:
     VectorObservationSize: 33
-    NumStackedVectorObservations: 1
+    NumStackedVectorObservations: 3
-  m_Model: {fileID: 11400000, guid: 66e38dc39b8474661869e59a21ed2837, type: 3}
+  m_Model: {fileID: 11400000, guid: d7bdb6a78154f4cf99437d67e4a569a8, type: 3}
   m_InferenceDevice: 0
   m_BehaviorType: 0
   m_BehaviorName: Reacher

Project/Assets/ML-Agents/Examples/Reacher/Prefabs/NewAgent.prefab

   m_EditorClassIdentifier: 
   m_BrainParameters:
     VectorObservationSize: 64
-    NumStackedVectorObservations: 1
+    NumStackedVectorObservations: 3
-  m_Model: {fileID: 11400000, guid: 66e38dc39b8474661869e59a21ed2837, type: 3}
+  m_Model: {fileID: 11400000, guid: e12acd64209f9468c899b9708b2702c3, type: 3}
   m_InferenceDevice: 0
   m_BehaviorType: 0
   m_BehaviorName: Reacher

Project/Assets/ML-Agents/Examples/Reacher/Scenes/ReacherNew.unity

       propertyPath: m_LocalEulerAnglesHint.z
       value: 0
       objectReference: {fileID: 0}
+    - target: {fileID: 114731167133171590, guid: 1c8be2c419f5747588fa6fdbdbaac538,
+        type: 3}
+      propertyPath: m_BrainParameters.NumStackedVectorObservations
+      value: 3
+      objectReference: {fileID: 0}
     m_RemovedComponents:
     - {fileID: 114955921823023820, guid: 1c8be2c419f5747588fa6fdbdbaac538, type: 3}
   m_SourcePrefab: {fileID: 100100000, guid: 1c8be2c419f5747588fa6fdbdbaac538, type: 3}