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/IPageSegmenter.cs

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+using System.Collections.Generic;
+using UglyToad.PdfPig.Content;
+
+namespace UglyToad.PdfPig.DocumentLayoutAnalysis
+{
+    /// <summary>
+    /// Page segmentation divides a page into areas, each consisting of a layout structure (blocks, lines, etc.).
+    /// <para> See 'Performance Comparison of Six Algorithms for Page Segmentation' by Faisal Shafait, Daniel Keysers, and Thomas M. Breuel.</para>
+    /// </summary>
+    public interface IPageSegmenter
+    {
+        /// <summary>
+        /// Get the text blocks.
+        /// </summary>
+        /// <param name="pageWords">The words to generate text blocks for.</param>
+        /// <returns>A list of text blocks from this approach.</returns>
+        IReadOnlyList<TextBlock> GetBlocks(IEnumerable<Word> pageWords);
+    }
+}