Customizability of CUDS containment

[yoavnash]

Instead of simply relying on the notion of "active relationships" to define the containment relationship in CUDS, it could be useful to extend it, to make it more flexible.

For example, for datasets, containment could be defined, so that the container contains only objects that the scientist chose. Then, when retrieving the dataset, only the original items are being extracted from the triplestore.

We might want to support multiple ways to define containment relationships at the same time.

Related to #636