Implement Userfiles registered as subsets of other Userfiles (Redmine 733)

[prioux]

(Created April 2010; originally marked as low priority)

In order to model more complex data structures as actually managed by scientists, we should be able to register as a Userfile a directory entry (file or directory) that is already a subset of an existing FileCollection. And recursively. (Assuming only the leafs can be SingleFiles, and all 'upper' nodes are all FileCollections).

Notes:

• Synchronization can result in duplication of data until we implement partial synchronization;
• Userfile deletion can result in cascades of more deletions (just like a 'rm -rf');
• All such 'embedded' Userfiles must belong to the same Data Provider, of course;
• This would be a good candidate to re-assign our tree-view code to show the 'subtree-of' relationships, instead of the faulty 'created from'; we could have 'hide/show' arrows to expand the tree structure;
• We would need to implement all the management operations to support data extraction, move, copy, delete, etc, along with making sure our data size values are accurate.