Fix: Cache used colors

* This greatly increases performance (Many orders of magnite faster)
* ColorsUsed is implemented so that every time it is queried the value
  is calculated on the fly
* When writing ColorsUsed I did not think this would be a problem since
  we only need it once: When calculating the color
* But the actual implementation used it at two points so that for x
  colored nodes the ColorsUsed were calculated twice.
* Ideally this would be fixed by changing the ColorsUsed implementation
  in the -Graph classes. This might be fixed later so consider this
  commit a hotfix

src/SparseTransform/Convert/ColoredDotWriter.cs

@@ -8,16 +8,16 @@ namespace SparseTransform.Convert
     /// </summary>
     public class ColoredDotWriter : DotWriter
     {
-        private IGraph _coloredGraph;
         private bool _textcolor;
-       
+        private int _colorsUsed;
+
         /// <summary>
         /// Constructor constructing a color-dictionary for coloring purpose.
         /// </summary>
         /// <param name="textColor">decision whether the labels should be colored or labelled.</param>
         public ColoredDotWriter(IGraph graph, bool textColor)
         {
-            this._coloredGraph = graph;
+            this._colorsUsed = graph.ColorsUsed;
             this._textcolor = textColor;
         }
        
@@ -30,7 +30,7 @@ namespace SparseTransform.Convert
         public override String NodeLabel(GraphNode node, String prefix)
         {
             // This is the offset. Less colors = more distance between
-            int hueFactor = 255 / _coloredGraph.ColorsUsed;
+            int hueFactor = 255 / this._colorsUsed;
             // Calculate the hue. We modulo with 255 to not exceed spectrum. Then convert to float for DOT
             double hue = Math.Round((double)((node.Color * hueFactor) % 255) / 255, 3);
 
@@ -41,7 +41,7 @@ namespace SparseTransform.Convert
             // If current node is colored, apply either text label with color or set fill color
             if (node.Colored)
             {
-                if (_textcolor || _coloredGraph.ColorsUsed > 255)
+                if (_textcolor || this._colorsUsed > 255)
                 {
                     label.Append($", label=\"{prefix}{node.Index}\\n color:{node.Color}\"");
                 }