Remove InnerScoreDirector.explainScore() that returns String and move it into ScoreManager or ScoreExplenation.

Also remove the two AbstractScoreDirector.explainScore() methods.

The ScoreDirector impls shouldn't be responsible to construct a summary String.
They should probably be responsible for creating the ScoreExplenation, to avoid calling fireAllRules 3 times.

I think the ScoreExplenation can even construct the summary string lazely, upon request.