Make it possible to run stardis with a completely custom stellar_model

[jvshields]

Currently, while possible, it's harder than it should be to do something like specify a density profile for the star. I think it would be good to make it easy for the user to specify a custom stellar_model object, and run stardis using that. This would allow for very generic investigations of spectra from exotic and interesting systems that can't be well represented with a marcs or mesa model.