canonicalize fragments extracted from derivations; & other new CLI options.

- discodop treetransforms: --outputfmt=wordpos to convert treebank to
  tagged tokens of the form word/POS.
- discodop parser: take --tags option for giving already tagged input
- discodop parser: --mpp option instead of --mpd; effective with or
  without --ctf pruning.
- canonicalize fragments from derivations produced by
  extractfragments();
  when the same fragment occurs in derivations for different trees, they
  will now be equivalent.
- include lexical fragments with output from extractfragments()
- discodop treetransforms command: when given --slice,
  only construct trees that are within the slice.
- discodop grammar ptsg: take -s option for specifying the
  start symbol to avoid spurious warnings;
  fractions with floats are now accepted (0.25/2.5).