Refactor downloads to be more robust

[nebfield]

Historical approach:

• We had weird intermittent HTTPS problems originally so we switched to FTP downloads
• But the FTP is shared across the EBI and it's often busy (error 421)

New approach:

• HTTPS download preferred, retried up to 5 times
• FTP fallback attempted, retried up to 5 times

The downloads module started off simple but got a bit untidy as we added features, so I've tried to simplify the structure and remove redundant functions.

The main motivation for this PR is making pgsc_calc less likely to throw errors on the INTERVENE platform when the FTP is busy.

To do:

• checksums ✅
• set local download directory ✅
• include pgsc_calc version number in UA ✅
• fix tests ✅

[smlmbrt]

I marked this for @ens-lgil to review, as he has more expertise on the API & checksums

[nebfield]

Thanks for the review @ens-lgil 🤠