Create a unit test to demonstrate host model nested parallel dispatches

[jeff-cohere]

As a first step to prove to ourselves that our single-column library approach can work on the machines we're targeting, it would be nice to demonstrate a nested parallel dispatch for a GPU build. Specifically:

• The outer loop dispatches a parallel for loop that calls a virtual function on some polymorphic object with a specific "column index"
• The virtual function performs a parallel for loop, using the column index to do something

We believe that the outer loop here executes on the host (CPU) and not, say, on the device (GPU). If this is true, we don't need to worry about instantiating aerosol process objects on GPUs--they can live on the CPU, and their run/update methods will perform work on a GPU, as long as the views they manipulate are stored there.

This task requires that we can configure Haero to build on GPU-equipped systems properly.

[jeff-cohere]

This was closed by #83