Support displaying selected fields for CSV format

[DuoLife-QNL]

When calling get_string or get_html_string or get_latex_string and passing fields = ["field1", "field2"] as the parameter, the output strings in either format contain the given fields in fields only. However, when calling get_csv_string or get_json_string, that does not work. It seems that these two functions do not take this setting into consideration.