Returns the neighbors of closest vertex to this cell at the given level,
by appending them to "output". Normally there are four neighbors, but
the closest vertex may only have three neighbors if it is one of the 8
cube vertices.
Requires: level < this->level(), so that we can determine which vertex is
closest (in particular, level == MAX_LEVEL is not allowed).
Returns the neighbors of closest vertex to this cell at the given level, by appending them to "output". Normally there are four neighbors, but the closest vertex may only have three neighbors if it is one of the 8 cube vertices.
Requires: level < this->level(), so that we can determine which vertex is closest (in particular, level == MAX_LEVEL is not allowed).