Renamed a helper function for computing linear offsets within a
multidimensional volume.
Split selection types into classes, requiring patching only occur within
the same class. Also implemented points-to-bounding-box patching
(points-to-points still unimplemented).
Added optimized support for point selections to identity using sieving
(different from read_bp.c, which performs a seek/read for every point in
a PG; it remains to be seen whether this is more or less efficient).