Modify xml2illustrationdata script to download image files

[nickynicolson]

Behaviour should be controlled by an argument passed to the script.

• [x] Add arguments to script (in argparse) and in Makefile
• [x] Implement downloadImage method
• [x] Any any extra libraries to the requirements.txt file (if necessary)
• [x] Do test run in CI or locally and check that images are downloaded