Add dataset support to radar-output

[blootsvoets]

This way, it can make a snapshot of the data to make it available as a standalone immutable dataset. For example a ZFS filesystem would support that using the ZFS API, while a general filesystem would probably use a combination of copying and compressing data into a single artifact.

see also https://github.com/RADAR-base/RADAR-TrueNAS/issues/1