Base class for STRtree and SIRtree. STR-packed R-trees are described in:
P. Rigaux, Michel Scholl and Agnes Voisard. Spatial Databases With
Application To GIS. Morgan Kaufmann, San Francisco, 2002.
An R-tree created using the Sort-Tile-Recursive (STR) algorithm, described
in: P. Rigaux, Michel Scholl and Agnes Voisard. Spatial Databases With
Application To GIS. Morgan Kaufmann, San Francisco, 2002.