Implement get_latest_result_dataframe helper function

[TheEdgeOfRage]

The get_latest_result implementation parses json, which is slow for large results. The csv endpoint is a lot more efficient, especially if you just need to put the data into a pandas DataFrame. get_latest_result_dataframe is a helper function that calls download_csv and returns a DataFrame from the result.

[bh2smith]

I re-ran the failing test to see if it was just an intermittent failure.