exposed the halfedge-loop circulator

git-svn-id: http://openmesh.org/svnrepo/OpenMesh@1101 fdac6126-5c0c-442c-9429-916003d36597
added a halfedge-loop circulator unit test

git-svn-id: http://openmesh.org/svnrepo/OpenMesh@1102 fdac6126-5c0c-442c-9429-916003d36597