[geometry]: [index] added detail::bounds(ind, box) function, quadratic algorithm is able to handle nspheres as Indexables. [extensions] added algorithms required by rtree to handled nspheres as Indexables: centroid(sph), covered_by(sph,box), equals(sph,sph), expand(box,sph), [i]comparable_distance_near(pt,sph), [i]bounds(sph,box), cartesian nsphere_in_box strategy, center_view<Sph> registered as Point.