Add a new threshold to the dict (key = stat_name)

def add(self, stat_name, threshold_description):
        """Add a new threshold to the dict (key = stat_name)"""
        if threshold_description not in self.threshold_list:
            return False
        else:
            self._thresholds[stat_name] = getattr(self.current_module,
                                                  'GlancesThreshold' + threshold_description.capitalize())()
            return True