Flatten directory structure in download

[jl-wynen]

At ESS files are no longer all in a dataset's source folder. Instead, some files have an absolute path in DataFile. Those paths should not be preserved locally.

The simplest way to handle this is to simply remove the entire path and store all files directly in target. We can add more customisation options later if needed.

One problem with this approach is that there can be multiple files with the same name. In this case, the client should refuse to download the files and require the user to filter out any files with duplicate names.