Add matched_fraction scoring function

[torognes]

For sequence existence, it may be nice to add “matched_fraction” as a scoring function, which would be a little different from the existing functions. If there is a match with a sequence, the score would be (duplicate_count of that sequence in that repertoire) / (total duplicate_count of the repertoire); duplicate_count of sequence is thus ignored. When --ignore-counts is enabled, the score would be the same as if all duplicate_counts are set to 1; so 1 / total repertoire length. I think this scoring function only makes sense in existence mode.