Return the threshold dict.
        If stat_name is None, return the threshold for all plugins (dict of Threshold*)
        Else return the Threshold* instance for the given plugin

def get(self, stat_name=None):
        """Return the threshold dict.
        If stat_name is None, return the threshold for all plugins (dict of Threshold*)
        Else return the Threshold* instance for the given plugin
        """
        if stat_name is None:
            return self._thresholds

        if stat_name in self._thresholds:
            return self._thresholds[stat_name]
        else:
            return {}