WIP Create atlas-derived-datasets.xml

Skeleton for #438 
This URL points to a home page, which is the same for all datasets.  This means that people will loose the current dataset context they have been in.  We could just as well catalogue "ATLAS External Resources" to achieve this...

Would it be possible to point to something like `http://example.org/visualise?dataset=WPath/2014/2` so that people would stay in their browsing context?  

Also, in the past we thought with @fsocher to explore whether we couldn't embed some of the tools in the portal itself, like CMS is doing.  Would this be feasible?