cdec option "promise_power"

po::value<double>()->default_value(0)

Give more beam budget to more promising previous-pass nodes when pruning - but
allocate the same average beams.  0 means off, 1 means beam proportional to
inside*outside prob, n means nth power (affects just --cubepruning_pop_limit).
note: for the same poplimit, this gives more search error unless very close to 0
(recommend disabled; even 0.01 is slightly worse than 0) which is a bad sign and
suggests this isn't doing a good job; further it's slightly slower to LM cube
rescore with 0.01 compared to 0, as well as giving (very insignificantly) lower
BLEU.  TODO: test under more conditions, or try idea with different formula, or
prob. cube beams.


with urdu mt09 2pass 1st pass tuned w/ unigram, prelm_beam_prune=1 scale_prune_srclen

promise_power: BLEU rescoring time
0: 20.21 1.8s
0.01: 20.16 1.9s
0.1: 20.11 2.0s
(insignificant differences)

10: 19.7 1.1s
(faster but much worse bleu)