geometry.index.rtree: added experimental nearest query iterator, rtree::qbegin() and rtree::qend() methods modified to support it, cosmetic change in spatial query iterator (addressof() instead of &)

svn path=/trunk/boost/geometry/; revision=83968
geometry.index: test - added test for experimental nearest query iterator, examples - added experimental nearest query iterator to benchmark.

svn path=/trunk/libs/geometry/; revision=83969
geometry.index.rtree: removed unused variable

svn path=/trunk/boost/geometry/; revision=83971