Add OneHot util to goal sensor

Project/Assets/ML-Agents/Examples/GridWorld/Prefabs/Area.prefab

   m_Script: {fileID: 11500000, guid: 163dac4bcbb2f4d8499db2cdcb22a89e, type: 3}
   m_Name: 
   m_EditorClassIdentifier: 
-  observationSize: 1
+  observationSize: 2
 --- !u!1 &1625008366184734
 GameObject:
   m_ObjectHideFlags: 0

Project/Assets/ML-Agents/Examples/GridWorld/Scenes/GridWorld.unity

   m_Script: {fileID: 11500000, guid: 163dac4bcbb2f4d8499db2cdcb22a89e, type: 3}
   m_Name: 
   m_EditorClassIdentifier: 
-  observationSize: 1
+  observationSize: 2
 --- !u!1 &260425459
 GameObject:
   m_ObjectHideFlags: 0

Project/Assets/ML-Agents/Examples/GridWorld/Scripts/GridAgent.cs

         Array values = Enum.GetValues(typeof(GridGoal));
         int goalNum = (int)gridGoal;
         goalSensor = this.GetComponent<GoalSensorComponent>();
-        goalSensor.AddGoal(goalNum);
+        goalSensor.AddOneHotGoal(goalNum, values.Length);
     }
 
     public override void WriteDiscreteActionMask(IDiscreteActionMask actionMask)

Project/Assets/ML-Agents/Examples/SharedAssets/Scripts/GoalSensorComponent.cs

             goalSensor.AddObservation(goal);
         }
     }
+
+    public void AddOneHotGoal(int goal, int range)
+    {
+        for (var i = 0; i < range; i++)
+        {
+            goalSensor.AddObservation(i == goal ? 1.0f : 0.0f);
+        }
+    }
 }
 
 public class GoalSensor : VectorSensor