fn writer<P>(&mut self, path: P, key: &[u8]) -> io::Result<BoxedWriter>
        where P: AsRef<Path>
    {
        let unique_value = self.unique_value(key);
        self.template.writer(path.as_ref(), &unique_value)
    }
Create a CSV writer for `key`.  Does not add headers.
src/cmd/partition.rs