output_columns not used in DataFrameConverter

[igorbrigadir]

The command line behavior is fine, but when using DataFrameConverter as a library in python code, setting output_columns effectively does nothing,

https://github.com/DocNow/twarc-csv/blob/main/dataframe_converter.py#L530

because that gets applied in

https://github.com/DocNow/twarc-csv/blob/main/csv_writer.py#L70

instead. Internally, DataFrameConverter always works with the full set of columns. Not sure if this is desirable or counter intuitive enough to change, it probably should be outputting the restricted set to be less awkward.