RR-transform, switch prob. models, shortest deriv. improvements

- relational-realizational parsing grammar transform, uses and produces
  morphological information, grammatical functions, and word order
  variation.
- adding & switching probability model of Grammar object:
  logprob & shortest derivation model are now simply a different set of
  probabilities for the same grammar.
- all of the shortest derivation parsing methods are now compatible with
  PCFG charts.
  - shortest: read off probabilities directly from derivation
  - sl-dop: factored-out tree parsing into separate function