data access for virtual collections

[ferrighi]

Some mmd records can express a virtual collection, i.e. a collection of files or directories on a thredds server. The virtual collection does not have a data access represented by the vocabulary currently in use (http, opendap, etc). Currently such records use the related_information "Dataset landing page" to connect to the data. In vision of disconnecting landing pages from thredds, I would suggest we add a data_access type as for example "thredds catalogue" or more generically "virtual collection".

As for today the HTTP definition is "Direct access to the full data file. May require authentication, but should point directly to the data file or a catalogue containing the data." So in principle we could use HTTP for data access, but it creates ambiguity between a direct download and a resolving url.

[ferrighi]

This is covered now by the design of the landing pages which include a link to the children. I still think it would be useful to have a url pointing to the folder of the whole collection on thredds, but I close this for now. We can reopen if necessary.