Cache hue color offset

* This should theoretically improve performance as we only calculate it
  once instead of once for every colored node

src/SparseTransform/Convert/ColoredDotWriter.cs

@@ -10,6 +10,7 @@ namespace SparseTransform.Convert
     {
         private bool _textcolor;
         private int _colorsUsed;
+        private int _hueFactor;
 
         /// <summary>
         /// Constructor constructing a color-dictionary for coloring purpose.
@@ -19,6 +20,8 @@ namespace SparseTransform.Convert
         {
             this._colorsUsed = graph.ColorsUsed;
             this._textcolor = textColor;
+            // This is the offset for coloring. Less colors = more distance between. Cached for NodeLabel.
+            this._hueFactor = 255 / this._colorsUsed;
         }
        
         /// <summary>
@@ -29,10 +32,8 @@ namespace SparseTransform.Convert
         /// <returns></returns>
         public override String NodeLabel(GraphNode node, String prefix)
         {
-            // This is the offset. Less colors = more distance between
-            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);
+            double hue = Math.Round((double)((node.Color * _hueFactor) % 255) / 255, 3);
 
             StringBuilder label = new StringBuilder();
             // Append label name and general layout info