Adding R21 precision JetTruthLabeling definitions

This adds the first step towards R21 precision truth labels. The truth label definitions are expected to be final, but the W/Z still need to be updated. This also schedules kt-splitting scale moments for ungroomed truth jets. In addition, it cleans up a few minor readability issue, implements DOxygen comments, and consolidates some functionality into reusable functions.

Tagging @sschramm, @cdelitzs, @mswiatlo, @adattagu, @farooque, @jcantero, and @tnobe for comments.