Add functionality for projecting chrF3 from confidence and estimating usability scores

[mshannon-sil]

As part of the early NT process, we're planning to estimate chrF3 from confidence scores, and then estimate the usability of each chapter and book based on the projected verse chrF3 scores and a usability distribution over chrF3 determined beforehand. Functionality needs to be added to SILNLP to handle this automatically, as it's been a partially manual process up until now.