'Return a list of features in the review. Each feature is a tuple made of
the specific item feature and the opinion strength about that feature.
:return: all features of the review as a list of tuples (feat, score).
:rtype: list(tuple)'

def features(self):

    features = []
    for review_line in self.review_lines:
        features.extend(review_line.features)
    return features
