create_physicsmodel creates different outputs for nsubs>1 vs nsubs==1

[Stargrazer82301]

For instance, create_physicsmodel creates an fgrid_subnames.txt file when nsubs>1, but not when nsubs==1. This means that significant code rewrites are needed to adjust production code from nsubs>1 to nsubs==1 (or vice-a-versa).

Maybe this is okay, but is antithetical to being able to do things consistently between targets/projects.