code: public GrammaticalRelation getGrammaticalRelation(int govIndex, int depIndex) {
      TreeGraphNode gov = getNodeByIndex(govIndex);
      TreeGraphNode dep = getNodeByIndex(depIndex);
      return getGrammaticalRelation(gov, dep);
    }
docs: Get GrammaticalRelation between gov and dep, and null if gov  is not the
  governor of dep