FileTransfer that links files

[jl-wynen]

If Scitacean is running on a system that has direct access to the remote file server, there is no need to download files. Instead, we can simply create a symlink or even set local_path = remote_path without creating a symlink.

A potential drawback is that if the file is mutable, the modifications to the 'downloaded' file affect the remote one. But files in SciCat should normally be read-only, so this might not actually be an issue.

Not sure how to handle uploads. We could just copy a file to the 'remote' path or maybe procure a location on 'remote' and write the file directly there such that the file transfer does nothing.