'Yields the most frequent class in the training data,
or a user-defined class if Classifier(baseline != MAJORITY).'

@property
def baseline(self):

    if (self._baseline not in (MAJORITY, FREQUENCY)):
        return self._baseline
    return ([(0, None)] + sorted([(v, k) for (k, v) in self._classes.items()]))[(-1)][1]
