Make ColorsUsed a public attribute of IGraph

* This should have been part of IGraph already since both graph
  implementations provide it.
* We also need it to be public to get the number of colors used in
  ColoredDotWriter

src/SparseTransform/DataStructures/AdjacencyGraph.cs

@@ -37,7 +37,7 @@ namespace SparseTransform.DataStructures
         /// <summary>
         /// Number of colors used for coloring.
         /// </summary>
-        private int ColorsUsed
+        public int ColorsUsed
         {
             get
             {

src/SparseTransform/DataStructures/BipartiteGraph.cs

@@ -69,7 +69,7 @@ namespace SparseTransform.DataStructures
         /// <summary>
         /// Number of colors used for coloring. Works if either or both sides are colored
         /// </summary>
-        private int ColorsUsed
+        public int ColorsUsed
         {
             get
             {

src/SparseTransform/DataStructures/IGraph.cs

@@ -6,6 +6,8 @@ namespace SparseTransform.DataStructures
     /// </summary>
     public interface IGraph
     {
+        public int ColorsUsed { get; }
+
         /// <summary>
         /// Adds an edge between nodes i and j. If either of the nodes is missing it is created.
         /// </summary>