Improve support of related_dataset

[ferrighi]

The mmd specification for related_dataset should be expanded to support different type of "relation_type". Possible relevant types can be: IsSupplementTo (for supporting datasets), IsPartOf (for collections - this could be a collection of parent datasets), IsDerivedFrom (to relate to sources). Extension of this element supports extraction of provenance information.

[steingod]

This would be a huge step forward and make the linked data approach much better in our system.

[mortenwh]

Where are we with this issue? The suggestions look good to me - is there anything that needs further discussion before implementation?