Error out when combining fieldsets of different mesh types

[VeckoTheGecko]

I missed the review phase on #2761

I think that we should make it an explicit error rather than a warning. I don't see a usecase for mixing fields since they're fundamentally different contexts

Thoughts @wyatt-fluidnumerics ?

(Thanks for the original PR btw! :) )

[VeckoTheGecko]

This also has the nice consequence of being able to expose a fieldset.mesh item (which is just the same as all the models one's), hence we can also remove the mesh argument from add_constant_field (since it always will be the same as the underlying models)

[wyatt-fluidnumerics]

Yeah this makes sense to me. I can't think of a use case for mixing fields either.