Normalize BM25 and TF-IDF scores

[davidmezzetti]

BM25 and TF-IDF scores are unbounded. They will continue to grow as the match improves. This change adds a flag to normalize scores from 0 - 1.

This method will calculate an average score using the average document length, average frequency and average idf score. A max score will then be calculated as 4 * average score. Lastly, scores will be scaled between 0 to 1 using the range of 0 to max score.

Formulas for this shown below.

average = score(average frequency, average idf, average document length)
max = 4 * average
score = score / max