Read data directly from various mounts

For internal use, it makes little sense to have to push the data on the system before being able to use it (it duplicates the raw data filesystem). Therefore, the following sources of data should be directly accessible through mounts:

• Local mounts (bind)
• SMB mounts
• sshfs (useful for testing)
• Amazon S3 bucket, see issue #9
• Nextcloud (FileDAV)

Please keep in mind that not all these items have the same priority!! The task might be reverted to a lower priority once SMB mounts will have been implemented...