In de Modflow6Simulation.split method we hardcode the "solver" key for the solver. This is wrong, as users are free to assign their solver package to any key. I think only the packages "time_discretization" and exchanges keys can be hardcoded, as they are generated by methods.

Therefore, we should change:

        new_simulation["solver"]["modelnames"] = xr.DataArray(
            list(get_models(new_simulation).keys())
        )

to:

        ims_key = simulation._get_pkgkey("ims")
        new_simulation[ims_key]["modelnames"] = xr.DataArray(
            list(get_models(new_simulation).keys())
        )

This requires adding a _get_pkgkey method to the Modflow6Simulation class as well, or moving the logic to utility function:

get_pkg_key(model_or_simulation, package_type)