Do not output duplicated vertices from QuadEdgeSubdivision class (#705)

Includes testcase

git-svn-id: http://svn.osgeo.org/geos/trunk@4007 5242fede-7e19-0410-aef8-94bd7d2200fb
Add some tolerance to VoronoiDiagram tester

git-svn-id: http://svn.osgeo.org/geos/trunk@4008 5242fede-7e19-0410-aef8-94bd7d2200fb