Squash self.out into string.

        Join every line in self.out with a new line and write the
        result to the output file.

def build_output(self, fout):
        """Squash self.out into string.

        Join every line in self.out with a new line and write the
        result to the output file.
        """
        fout.write('\n'.join([s for s in self.out]))