CSV outputted with Headers as first set of results.

def _init(self):
        """CSV outputted with Headers as first set of results."""
        # Note that we assume params.json was already created by JsonLogger
        progress_file = os.path.join(self.logdir, "progress.csv")
        self._continuing = os.path.exists(progress_file)
        self._file = open(progress_file, "a")
        self._csv_out = None