[download] Resume download of a dataset

[aaperis]

As a user, I want to be able to resume downloading a dataset so that I don't have to download all the files again if my download job is interrupted.

Ad a --continue flag to do this (or --resume would be fine as well)

Acceptance criteria

• Download of files that exist locally is skipped provided that these have the same name.
• Tests verifying the changes are added

Additional context

Estimation of size: medium

Estimation of priority: low