@Override
    public Optional<SignificanceModel> getModel(Language language) {
        if (!models.containsKey(language))
        {
            return Optional.empty();
        }
        return Optional.of(models.get(language));
    }
Default implementation of {@link SignificanceModelRegistry}.
This implementation loads models lazily and caches them.

@author MariusArhaug
linguistics/src/main/java/com/yahoo/language/significance/impl/DefaultSignificanceModelRegistry.java