Function csv_challenge::write_csv

pub fn write_csv(csv_data: &str, filename: &str) -> Result<(), Error>

Usage:

let filename = PathBuf::from("./files/challenge.csv");
let csv_data = load_csv(filename).unwrap();
let modified_data = replace_column(csv_data, "City", "Beijing").unwrap();
let output_file = write_csv(&modified_data, "output/test.csv");
assert!(output_file.is_ok());