Better data access metadata

[bpedersen2]

Currently, the metadata for accessing the data require quite some extra knowledge to get access to the files.
It would be good if we can carry the following informations per dataset (or [orig]datablock):

• access protocol: ssh, sftp, s3 , web-downloader ,globus? ....
• storage host
• access host if different from storage host?
• base path on storage

I would imagine that even more than one option can be supported ( -> list of options)

Related to SciCatProject/scitacean#37

The direct link option should probably not get added, as it is only usable in internal systems.