Convert UTF8 characters in a CSV file or dataframe into ASCII

    Args:
      df (DataFrame or str): DataFrame or path or url to CSV

def clean_df(df, header=None, **read_csv_kwargs):
    """ Convert UTF8 characters in a CSV file or dataframe into ASCII

    Args:
      df (DataFrame or str): DataFrame or path or url to CSV
    """
    df = read_csv(df, header=header, **read_csv_kwargs)
    df = df.fillna(' ')
    for col in df.columns:
        df[col] = df[col].apply(unicode2ascii)
    return df