'Return a named statistic collected during training, or a dictionary of all
available statistics if no name given
:param statistic: name of statistic
:type statistic: str
:return: some statistic collected during training of this tagger
:rtype: any (but usually a number)'

def train_stats(self, statistic=None):

    if (statistic is None):
        return self._training_stats
    else:
        return self._training_stats.get(statistic)
