Support WRF nested grids in LILAC

We will need to extend LILAC to support nested WRF grids.

Here are some notes from a discussion of this 2019-02-28 (https://github.com/ESCOMP/CTSM/wiki/Meeting-Notes-2019-Software#nested-wrf-domains):

> How could we handle having nested domains in CTSM?
>
> The problem is that we have a lot of module-level data. So if multiple
> nests share processors (which they do, in practice), we'd run into
> trouble.
>
> Noah-MP handles this by rereading its parameter structure each time it
> starts up a given nest.
>
> We may want to consider packaging all of our module-level variables so
> that we can have separate packages for each part of the nest.
>
> (Note that we can't simply create a single unstructured grid that
> smushes together all of the nest grids, because they could have
> different time steps, and maybe other parameters.)

If it wasn't for the last parenthetical, an unstructured grid might be a reasonable option. Since CTSM has a different decomposition from the atmosphere anyway, we may be able to get around the problem of having module-level data mentioned above. So we could use the multi-instance capabilities for this. In that case, the main work may be in LILAC - supporting multiple CTSM instances.