Download the file represented by the download_key.

def download(self, download_key, raise_exception_on_failure=False):
        """Download the file represented by the download_key."""
        query = {"output": "json", "user_credentials": self.api_key}
        resp = requests.get(
            "%sdownload/%s" % (self._url, download_key),
            params=query,
            timeout=self._timeout,
        )

        if raise_exception_on_failure and resp.status_code != 200:
            raise DocumentDownloadFailure(resp.content, resp.status_code)
        return resp