Validate boundary_conditions argument to model constructors

[glwagner]

Cause stuff like

model = HydrostaticFreeSurfaceModel(; grid, coriolis, boundary_conditions = (; u_bcs))

doesn't fail, it just doesn't actually apply boundary conditions to u (which was intended here).

In the past we've allowed some fairly complex input. So maybe we can just pass a warning when keys of boundary_conditions are not prognostic fields. Even more neutral would be error if none of the keys are prognostic fields (which would still catch the issue above but could miss some other annoying typos).