Add no_answer option to results

[tholor]

So far we applied the n_ans_threshold on FARM side, but then dropped all no_answer results in the final aggregation step across all candidate paragraphs (in haystack):

We should add it to the results. There are a few different options how to go with this, but I would suggest:

• Take the no_answer candidate from the paragraph with the best "real answer" (as both are really competing for the top 1 answer)
• Use as score: score of no_answer - no_answer_threshold (as no_answer_thresold is kinda boosting the score and this should be reflected in the final results, where we want to sort by score)

Adressing #23