Extension / propagation of a point through z stack (expand_along_z) is limited

[vreuter]

It's obvious that the floor is 0, but what should the upper bound on z be? Right now, we use the max coordinate from among the empirical pool (observed points), but the voxel stack may well be taller/longer than that (e.g., it's 12 slices, and max z coordinate observed is only 9, so then the last couple of z slices would never have any indication of a point).