UTests pass with Pathfinder2D

PathFinding/IPlanarCollection.cs

@@ -1,7 +1,10 @@
-namespace PathFinding
+using System.Collections.Generic;
+
+namespace PathFinding
 {
-  interface IPlanarCollection<T>
+  public interface IPlanarCollection<T> : IEnumerable<T>
   {
     T this[int x, int y] { get; set; }
+    int GetLength(int dimension);
   }
 }