Added IElement interface.

This is for the conviencnce of constraining nodes that won't have children.

Author: Oren (electricessence)

IElement.cs

@@ -0,0 +1,8 @@
+using System;
+namespace Open.Hierarchy
+{
+	public interface IElement<T>
+	{
+		T Value { get; set; }
+	}
+}

Node.cs

@@ -6,7 +6,7 @@
 
 namespace Open.Hierarchy
 {
-	public sealed partial class Node<T> : INode<Node<T>>
+	public sealed partial class Node<T> : INode<Node<T>>, IElement<T>
 	{
 		public Node<T>? Parent { get; private set; }
 		object? IChild.Parent => Parent;