Write ps1 script to download data

[psh0078]

Fix #880

Write a script that checks if data files already exist and if their checksum matches the expected checksum. If the files do not exist or their checksum does not match, the script will download the required zip files.

[rmunn]

A pretty large change requested here, sorry. But my intent, which I may not have explained well enough, was that the script would take parameters, and the hash values (and URLs) would be hardcoded in the Taskfile.yml file, not in the script.

[rmunn]

Very good. This is ready to merge now.

[rmunn]

Note that this is going to create merge conflicts with #888 which adds a third project. That's fine; we'll just resolve the merge conflicts in #888 once this is merged.