The subvolume to be read is controlled by additional parameter 'extent' passed to function dataset(). The syntax of this parameters is extent=(xmin, xmax, ymin, ymax, zmin, zmax) for 3D snapshots or extent=(xmin, xmax, ymin, ymax) for 2D snapshots. Signed-off-by: Grzegorz Kowal <grzegorz@amuncode.org>