Support function for write_csv

def _csv_str(self, param, stats, quantiles, index=None):
        """Support function for write_csv"""

        buffer = param
        if not index:
            buffer += ', '
        else:
            buffer += '_' + '_'.join([str(i) for i in index]) + ', '

        for stat in ('mean', 'standard deviation', 'mc error'):
            buffer += str(stats[stat][index]) + ', '

        # Index to interval label
        iindex = [key.split()[-1] for key in stats.keys()].index('interval')
        interval = list(stats.keys())[iindex]
        buffer += ', '.join(stats[interval].T[index].astype(str))

        # Process quantiles
        qvalues = stats['quantiles']
        for q in quantiles:
            buffer += ', ' + str(qvalues[q][index])

        return buffer + '\n'