Changes to download v4 tournament data with current_round label

[AndyJ74]

Code added to download data not included in current example_model_advanced.py script. Otherwise script won't finish successfully unless data has already been download manually or by running another script that happens to have done this with the same naming convention.

Added {current_round} tag/label to data file names as descriptions of v4 data at 'https://numer.ai/data/v4' states that these files will be changed or added to weekly. This way the script will run with latest data if called again in following weeks.