Support multi-variable calculations with multiple data sources

[spencerahill]

A crude version of this once existed but then lapsed, and its logic was ultimately removed in #178 (c.f. #117). Nevertheless, this is something we do want to support eventually.

A common use-case in this realm (for me at least) is decomposing changes in atmospheric tracer transports in response to some forcing into "dynamic" and "thermodynamic" components. The former uses thermodynamic variables from the control simulation multiplied by the change (perturbation minus control) in the winds. The latter uses the change in the thermodynamic variables and the control winds.

It is far from straightforward how to do this right. Among other things, it is not clear how to store the results of these calculations to disk, since the proj/model/run/variable directory structure no longer fits. So much further discussion is needed on this. (I don't have time to do so right now but wanted to get this stub in to track this while it's fresh on my mind from #178).