Lenskit: Popularity baseline (Learning to Rank)

This post is dedicated to popularity baseline in Lenskit 2 framework. The framework lacks this baseline. I therefore provide an implementation and demonstrate the results of the baseline.
The implementation includes three classes:
PopItemScorer - items scorer, which provides actual scores for items
PopModel - model that contains popularity of each item
PopModelBuilder - builder calculates popularity for each items and puts them to the model.

Lenskit code example

Recently, I started using Lenskit framework. The framework is designed for recommendations. It contains a few useful recommendation algorithms, such as item-item collaborative filtering and matrix factorization. However, there is a lack of documentation and examples on the framework.
I needed to use SimpleEvaluator class and I could not find a relevant documentation on the the class or a good example how I can use it.