DDM Assessment: User Stories, Rucio

BH: as part of the DDM assessment work we need to document the way that each DDM option (SI/ Rucio) might be able to support each user story. In some cases, this can be proven by example (from an existing deployment within or external to the SRCNet), in others it may be by a description of how, in principle, the use case may be achieved, or perhaps a description of what is missing in order to enable the use case.