Variable selection from the variable menu should return collection options

[bnlawrence]

After faceted browse, the user will see a list of variables. When they select one of those, there still may be several different atomic datasets inherent in the results. We need users to be able to see
a) what they have in common, and
b) how they differ, and
And then they should be able to
c) select an atomic dataset.

We need to come back to the notion of atomic datasets. At which point we might be able to identify quarks (which may differ eg. with creation date or lcoation).