Make editor extensions and inference utils internal (#3390)

* make our editor extensions internal

* MLAgents.InferenceBrain.Utils too

com.unity.ml-agents/Editor/AgentEditor.cs

     */
     [CustomEditor(typeof(Agent), true)]
     [CanEditMultipleObjects]
-    public class AgentEditor : Editor
+    internal class AgentEditor : Editor
     {
         public override void OnInspectorGUI()
         {

com.unity.ml-agents/Editor/BehaviorParametersEditor.cs

     */
     [CustomEditor(typeof(BehaviorParameters))]
     [CanEditMultipleObjects]
-    public class BehaviorParametersEditor : Editor
+    internal class BehaviorParametersEditor : Editor
     {
         const float k_TimeBetweenModelReloads = 2f;
         // Time since the last reload of the model

com.unity.ml-agents/Editor/BrainParametersDrawer.cs

     /// Inspector.
     /// </summary>
     [CustomPropertyDrawer(typeof(BrainParameters))]
-    public class BrainParametersDrawer : PropertyDrawer
+    internal class BrainParametersDrawer : PropertyDrawer
     {
         // The height of a line in the Unity Inspectors
         const float k_LineHeight = 17f;

com.unity.ml-agents/Editor/DemonstrationDrawer.cs

 /// </summary>
 [CustomEditor(typeof(Demonstration))]
 [CanEditMultipleObjects]
-public class DemonstrationEditor : Editor
+internal class DemonstrationEditor : Editor
 {
     SerializedProperty m_BrainParameters;
     SerializedProperty m_DemoMetaData;

com.unity.ml-agents/Editor/DemonstrationImporter.cs

     /// Asset Importer used to parse demonstration files.
     /// </summary>
     [ScriptedImporter(1, new[] {"demo"})]
-    public class DemonstrationImporter : ScriptedImporter
+    internal class DemonstrationImporter : ScriptedImporter
     {
         const string k_IconPath = "Assets/ML-Agents/Resources/DemoIcon.png";
 

com.unity.ml-agents/Runtime/InferenceBrain/Utils/Multinomial.cs

     /// entry[i] = P(x \le i), NOT P(i - 1 \le x \lt i).
     /// (\le stands for less than or equal to while \lt is strictly less than).
     /// </summary>
-    public class Multinomial
+    internal class Multinomial
     {
         readonly System.Random m_Random;
 

com.unity.ml-agents/Runtime/InferenceBrain/Utils/RandomNormal.cs

     /// https://en.wikipedia.org/wiki/Marsaglia_polar_method
     /// TODO: worth overriding System.Random instead of aggregating?
     /// </summary>
-    public class RandomNormal
+    internal class RandomNormal
     {
         readonly double m_Mean;
         readonly double m_Stddev;