Make TestTool.samplePolygon() be able to accept null input
Add Polygon2D.getVertes() and make a test for Segment's pred links

 - pass test Polygon2DTest.testPolygonSegmentPredLink()
Add Segment2D.midPoint(double[]) and Segment2D.subdivide()
Add Polygon2D.fractionize(double) and test it

 - test by Polygon2DTest.testFractionize()
Add DoubleArrayComparator.comparatorsForAll(int)
Add Math2D.isSegmentsIntersecting() and test
Add Polygon2D.rangeSearch()
Make Boundary2D implement IntIdentity
let all the Segment2D of Polygon2D have ids

 - every Segment2D instance id is identical to others' in the same Polygon2D
add static pack() operation in PairPack and replace codes in LayerRangeTree

 - Test by LayeredRangeTreeTest
Add IntIdentityComparator
Make Model2D and Model.search()

 - test priliminarily by Model2DTest