Returns 3 arrays for traversing the taxonomy:
- {@code parents}: {@code parents[i]} denotes the parent of category ordinal {@code i}.
- {@code children}: {@code children[i]} denotes a child of category ordinal {@code i}.
- {@code siblings}: {@code siblings[i]} denotes the sibling of category ordinal {@code i}.
To traverse the taxonomy tree, you typically start with {@code children[0]} (ordinal 0 is reserved for ROOT), and then depends if you want to do DFS or BFS, you call {@code children[children[0]]} or {@code siblings[children[0]]} and so forth, respectively.
NOTE: you are not expected to modify the values of the arrays, since the arrays are shared with other threads. @lucene.experimental
