properly reverse morphology='add'; morph/lemma node transformation

- when morphology='add' is used, the result is now splitted and the POS
  tag and morphology tag are exported separately when converted to
  export format.
- the treebank options morphology='between' and lemmas='between',
  insert morphology/lemma nodes;
  the transformations MORPH-NODE and LEMMA-NODE do the same but are
  reversible and store the morphology / lemma in the converted tree.