Support for subgrid for generating AST inputs

[galaxyumi]

It is related to the closed issue (#374).

In the current BEAST, some runs (e.g., generating physics model grid, fitting) support the subgrid functionality, while some runs (e.g., generating AST inputs, trimming?) don't. Because most users want to run the BEAST all the way through from generating physics model grid to fitting, the subgrid functionality should be either enabled or disabled all the way through for consistency.

[karllark]

@lea-hagen : thoughts?

[lea-hagen]

ASTs: We decided in #374 to just do a coarse grid (I just put an example in https://github.com/BEAST-Fitting/beast-examples/pull/4). Because the SEDs are randomly chosen, we'd have to read all of the subgrids into memory, which would be difficult or impossible. But other strategies would be welcomed!

Trimming: This does support subgrids. It reads in datamodel and grabs n_subgrid from there. The other scripts in tools/run/ should be updated to do the same thing, but I wrote them before moving the n_subgrid parameter into datamodel, so they have the subgrids as a function input instead.