Document Layout Analysis - IPageSegmenter, Docstrum

- Create a TextBlock class
- Creates IPageSegmenter
- Add other useful distances: angle, etc.
- Update RecursiveXYCut
 - With IPageSegmenter and TextBlock
 - Make XYNode and XYLeaf internal
- Optimise (faster) NearestNeighbourWordExtractor and isolate the clustering algorithms for use outside of this class
- Implement a Docstrum inspired page segmentation algorithm

src/UglyToad.PdfPig/DocumentLayoutAnalysis/XYNode.cs

@@ -8,7 +8,7 @@ namespace UglyToad.PdfPig.DocumentLayoutAnalysis
     /// <summary>
     /// A Node used in the <see cref="RecursiveXYCut"/> algorithm.
     /// </summary>
-    public class XYNode
+    internal class XYNode
     {
         /// <summary>
         /// Returns true if this node is a leaf, false otherwise.