changed operation in pipeline - added a grouping op.

this group occurres at the start of the transformation from translated
sentences to fol rules - it groups the translated sentences by the orig
sentence.

this has two benefits:
- it reduces the amount of work that has to be done if the original
  sentence cannot be converted all the way to CNF (which may actually
  occur!)
- it forces the getCNF calls to be run during a map stage, because the
  previous map-reduce task is ended with the groupBy call.