added comments and tests

HOMEWORK6/HOMEWORK5/HOMEWORK-4/SpreadsheetEngine/ExpressionTree.cs

@@ -10,6 +10,9 @@ namespace SpreadsheetEngine
     using System.Text;
     using System.Threading.Tasks;
 
+    /// <summary>
+    /// ExpressionTree Class 
+    /// </summary>
     public class ExpressionTree
     {
         private Node root;