Calculate basic alignment score for word pairs from the sentence_score

[pmachapman]

This PR takes the average of the forward and reverse sentence_score from eflomal, places it between 0 and 1.0, and saves it for each word pair score.

Not accurate at all as a per-word pair score, but without using a different aligner than eflomal, I am not too sure how else to get an accurate word alignment score.

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This change is 

[Enkidu93]

That's too bad that they don't expose the per-word scores - I didn't realize that, sorry. I leave it to Damien whether he's comfortable with this workaround until eflomal is properly integrated.

@Enkidu93 reviewed 2 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on ddaspit).