* removed code triplication in ManagedModelStore
* LTRScoringQuery.java tweaks
* FeatureLogger.makeFeatureVector(...) can now safely be called repeatedly (though that doesn't happen at present)
* make Feature.FeatureWeight.extractTerms a no-op; (OriginalScore|SolrFeature)Weight now implement extractTerms
* LTRThreadModule javadocs and README.md tweaks
* add TestFieldValueFeature.testBooleanValue test; replace "T"/"F" magic string use in FieldValueFeature
* add TestOriginalScoreScorer test; add OriginalScoreScorer.freq() method
* in TestMultipleAdditiveTreesModel revive dead explain test
Christine Poerschke
Michael Nilsson