Update Tokenizer method signature

[mh-northlander]

Currently some Tokenizer methods return MorphemeList. Update them to List<Morpheme>.

Return type should be specific i.e. MorphemeList, and argument type should be general i.e. List.

[mh-northlander]

We want to hide MorphemeList from outside.

Let's add Tokenizer.split(List<Morpheme>, mode) to replace MorphemeList.split(mode).