DRS update for compatability with ESGF

[bnlawrence]

Need to ensure that the DRS update is compatible with ESGF, and appropriate documentation on the relationships exist (source: conversation with PJK and future ESGF)

[philipkershaw]

David’s point (from chat log): At the moment there is no real concept of a "simulation" in the ESGF catalog. A simulation includes variables from the same model but from different realms. Also, we'd want the concept of an "ensemble" clearly defined, for example as a set of simulations meeting criteria (variable availability, model included in set of experiments). 
Finally, I think we need to improve linkages between simulations, for example the concept of "isdrivenby" is key for regional models.