geometry.index test: changed rtree parameters to ensure that rtree will have at least 3 levels, added test for experimental spatial query iterator.

svn path=/trunk/libs/geometry/; revision=83934
geometry.index: added experimental spatial query iterator and rtree::qbegin() and rtree::qend() methods, added allocator_type to Allocators.

svn path=/trunk/boost/geometry/; revision=83935