Add download_csv

[bh2smith]

Function will look like this

    def download_csv(self, job_id: str) -> ExecutionResultCSV:
        """
        Retrieves the CSV content of the execution results for `job_id`
        This is just an alias for `get_result_csv`
        """
        return self.get_result_csv(job_id)

but should come in only after #64 lands (the function renaming).

[bh2smith]

Closed by #73