correctly export frequencies with Bitpar format

- When exporting a PCFG to bitpar, the numerator from the fraction was
  used as its frequency, but fractions are simplified which distorts the
  original distrubution. The current fix uses a common denominator but
  does not retain the original frequencies of the treebank.
- When deciding whether to write integral frequenies or decimal
  fractions, take probability mass added by lexical smoothing into
  account (no normalization is done after lexical smoothing so the
  weights of the grammars are superficient).