Returns all neighbors of this cell at the given level. 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 returned more than once. There could be up to eight neighbors
including the diagonal ones that share the vertex.
Returns all neighbors of this cell at the given level. 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 returned more than once. There could be up to eight neighbors including the diagonal ones that share the vertex.
This requires level >= ci.level().