Alpino sample corpus; draw trees during parsing; etc.

- Replace Tiger sample `sample2.prm` with Alpino sample, which can be
  distributed under the LGPL.
- runexp: visualize parse trees during parsing, with matching
  bracketings highlighted.
- pcfg: disable assertions for illegal outside probabilities; needs to be
  investigated (probably underflow), but disabling the assertion allows
  parsing to go on with 'no parse' as result for the sentence.