Add an additional DATETIME column with standar datetime format.

    This currently manipulates the incoming DataFrame!

def add_datetime(dataframe, timestamp_key='UNIXTIME'):
    """Add an additional DATETIME column with standar datetime format.

    This currently manipulates the incoming DataFrame!
    """

    def convert_data(timestamp):
        return datetime.fromtimestamp(float(timestamp) / 1e3, UTC_TZ)

    try:
        log.debug("Adding DATETIME column to the data")
        converted = dataframe[timestamp_key].apply(convert_data)
        dataframe['DATETIME'] = converted
    except KeyError:
        log.warning("Could not add DATETIME column")