Allow parametrizing Viterbi decoding.

[foxik]

Allow parametrizing the Viterbi decoding algorithm.

[foxik]

Done in:

• c73853b: add viterbi_order which allows specifying decoding order smaller than model order
• ba8da1c: add viterbi_beam_size (beam size of a stage) and viterbi_stage_pruning (maximum increase in cost in every stage) All options can be specified in the configuration and also overriden as parameters of korektor executable

[foxik]

Using viberbi_beam_size of 15 and viterbi_stage_pruning of 2.5 decreases Korektor runtime by a factor of 2 (diacritization) and 3 (spellchecking) in korektor-czech-130202 model, while having no effect on the test data results. On other data it can have some effect, but it will be probably minor.

On the other hand, decreasing viberbi_order to 1 has a larger effect on result accuracy, so we are not using it for korektor-czech-130202. But it will be useful if we try models with higher order than 3.