Append all neighbors of this cell at the given level to "output". Two
cells X and Y are neighbors if their boundaries intersect but their
interiors do not. In particular, two cells that intersect at a single
point are neighbors. Note that for cells adjacent to a face vertex, the
same neighbor may be appended more than once.
Append all neighbors of this cell at the given level to "output". Two cells X and Y are neighbors if their boundaries intersect but their interiors do not. In particular, two cells that intersect at a single point are neighbors. Note that for cells adjacent to a face vertex, the same neighbor may be appended more than once.
REQUIRES: nbr_level >= this->level().