def strttime_np(datetime64):
    """
    datetime64: of type numpy.datetime64
    return: datetime in format '%Y%m%d_%H%M%S'
    equiv. to datetime.utcfromtimestamp(datetime64.astype(int)).strftime('%Y%m%d_%H%M%S')
    but faster by almost an order of magnitude
    """
    temp = str(datetime64)
    return temp[:4] + temp[5:7] + temp[8:10] + '_' + temp[11:13] + temp[14:16
        ] + temp[17:19]
