Don't require all 4 component models

[juliasloan25]

Currently, running a coupled simulation requires creating atmosphere, land, ocean, and ice models. This means that if we run a simulation with only one or two models (e.g. atmosphere and land), we still have to initialize all 4, then we zero out the area fraction of the unused models.

It seems pretty straightforward to transition to require atmos and >= 1 surface model. We also want to be able to run the atmosphere without any surface model, but we need to think a little bit more about what the fluxes would look like in this case, and similarly for 1 component model only.