Feature/download resume dataset

[aaperis]

Related issue(s) and PR(s)
This PR closes #405.

Description

If the flag -continue is given to download then any existing file with the same name and file path will be skipped during download.

How to test
See that the added integration tests pass.

[MalinAhlberg]

All looks well to me, and works fine when tested.

I get a bit confused by the name continue, however. It sounds like that without this flag, the program would stop when trying to download files that already exists. Maybe we could make this clearer in the docs? (sorry for not realizing and raising this earlier)

[aaperis]

All looks well to me, and works fine when tested.

I get a bit confused by the name continue, however. It sounds like that without this flag, the program would stop when trying to download files that already exists. Maybe we could make this clearer in the docs? (sorry for not realizing and raising this earlier)

I can see why it may seem confusing and I agree that this is not ideal. I wanted to avoid using -resume because this could be perceived as "resuming a partially downloaded file" whereas here this is not the case. We also use -continue for a similar functionality in the Upload module so I thought this would be at least consistent.
Given that this is the first iteration for implementing this functionality (I would prefer to check more than the filenames), I say we go with it for now, and I open an issue for a follow-up that includes the naming of the flag (#544).