Add lexicon-free label-synchronous beam-search algorithm

[SimBe195]

Implement a lexicon-free label-synchronous search algorithm (based on SearchAlgorithmV2). This is mostly similar to LexiconfreeTimesyncBeamSearch with the most notable differences being:

• Instead of a blank token, this search needs a sentence-end token.
• There is a distinction between terminated hypotheses and active hypotheses, depending on whether they have produced sentence-end yet.
• Since hypotheses may be terminated at different lengths, length normalization is introduced for score comparison.
• The search terminates when all hypotheses in the beam are terminated or when an upper limit of decoding steps has been reached. This limit depends on the number of input features that have been fed.

[larissakl]

I have just noticed that you have not registered this algorithm as a SearchTypeV2 in Search/Module yet. I would say that we should implement this directly when introducing a new search algorithm.