Add ability to select one or more variables to constrain search. Note, these variable selections should "flow through" to the file download page when working with multiple dataset search results.

From end user request email:

> - Multiple Variables: I'm doing an analysis that will involve comparing several variables across models & scenarios. I would like to be able to only see those data runs for which all of my variables are available. At present, it seems like I can only select 1 variable at a time.
An option to select several variables is also forthcoming, but it won't do logic such as *only* models for which A and B and C are available. If you ask for A and B and C, you'll see all data available for those variables, which in some cases will be incomplete.