Permits the Pair FE's to use getTopNgrams(): can be optionally overridden, to
constrain which ngrams are used as features. (Pair ngram FE's generate a huge number of
features, that must usually be constrained.) Without this method, getTopNgrams() must be
overridden in LucenePFEBase with essentially the same method, but with the constraint option
in place, resulting in code duplication.