Refactor remove unecessary imports; Add abstract modifier to Immutable and MutableVariable

Author: Fireball19

Packages/SOVariables/Runtime/IVariable.cs

@@ -1,5 +1,3 @@
-using UnityEngine;
-
 namespace RaCoding.Variables
 {
     public interface IVariable<T>

Packages/SOVariables/Runtime/Immutable/ImmutableVariable.cs

@@ -2,13 +2,13 @@
 
 namespace RaCoding.Variables
 {
-    public class ImmutableVariable<T> : ScriptableObject, IVariable<T>
+    public abstract class ImmutableVariable<T> : ScriptableObject, IVariable<T>
     {
         [SerializeField] private T value;
 
         public T Value 
         {
-            get { return this.value; }
+            get { return value; }
         }
     }
 }

Packages/SOVariables/Runtime/Mutable/MutableVariable.cs

@@ -2,14 +2,14 @@
 
 namespace RaCoding.Variables
 {
-    public class MutableVariable<T> : ScriptableObject, IVariable<T>
+    public abstract class MutableVariable<T> : ScriptableObject, IVariable<T>
     {
         [SerializeField] private T value;
         [SerializeField, ReadOnlyInInspector] private T resetValue;
 
         public T Value 
         {
-            get { return this.value; }
+            get { return value; }
             set { this.value = value; }
         }
 

Packages/SOVariables/Runtime/Mutable/Reference/MutableVariableReference.cs

@@ -2,7 +2,10 @@ namespace RaCoding.Variables
 {
     public abstract class MutableVariableReference<T> : IVariable<T>
     {
-        protected abstract MutableVariable<T> Reference { get; }
+        protected abstract MutableVariable<T> Reference 
+        { 
+            get; 
+        }
 
         public T Value
         {