make RayPerception.Perceive abstract and override in base classes (#2788)

UnitySDK/Assets/ML-Agents/Examples/SharedAssets/Scripts/RayPerception.cs

-using System.Collections.Generic;
+using System.Collections.Generic;
 using UnityEngine;
 
 public abstract class RayPerception : MonoBehaviour
-    public virtual List<float> Perceive(float rayDistance,
+    abstract public List<float> Perceive(float rayDistance,
-        float startOffset, float endOffset)
-    {
-        return m_PerceptionBuffer;
-    }
+        float startOffset=0.0f, float endOffset=0.0f);
 
     /// <summary>
     /// Converts degrees to radians.

UnitySDK/Assets/ML-Agents/Examples/SharedAssets/Scripts/RayPerception2D.cs

-using System.Collections.Generic;
+using System.Collections.Generic;
 using UnityEngine;
 
 namespace MLAgents
         /// <param name="rayDistance">Radius of rays</param>
         /// <param name="rayAngles">Angles of rays (starting from (1,0) on unit circle).</param>
         /// <param name="detectableObjects">List of tags which correspond to object types agent can see</param>
-        public List<float> Perceive(float rayDistance,
-            float[] rayAngles, string[] detectableObjects)
+        /// <param name="startOffset">Unused</param>
+        /// <param name="endOffset">Unused</param>
+        public override List<float> Perceive(float rayDistance,
+            float[] rayAngles, string[] detectableObjects,
+            float startOffset=0.0f, float endOffset=0.0f)
         {
             m_PerceptionBuffer.Clear();
             // For each ray sublist stores categorical information on detected object

UnitySDK/Assets/ML-Agents/Examples/SharedAssets/Scripts/RayPerception3D.cs

-using System;
+using System;
 using System.Collections.Generic;
 using UnityEngine;
 
         /// <param name="endOffset">Ending height offset of ray from center of agent.</param>
         public override List<float> Perceive(float rayDistance,
             float[] rayAngles, string[] detectableObjects,
-            float startOffset, float endOffset)
+            float startOffset=0.0f, float endOffset=0.0f)
         {
             if (m_SubList == null || m_SubList.Length != detectableObjects.Length + 2)
                 m_SubList = new float[detectableObjects.Length + 2];