Disallow meshes with different cell types in a `MeshSeq`

[joewallwork]

@ddundo: At the same time I wasn't sure if anyone would even want to do this. And if they do do it, assert self.finite_element.cell == mesh.coordinates.ufl_element().cell in Field.get_element would return an error. So if it really is super niche, perhaps I could just write a comment saying that this line assumes that all cell types are the same? And then we can address it in the future if needed?

@jwallwork23: Ah sorry, just realised I merged before this got resolved. I agree that we should just disallow having meshes with different cell types for simplicity. Indeed, it seems like a niche request and it would just cause us headaches I think.

Originally posted by @ddundo in #301 (comment)

[ddundo]

Thanks for opening this. Just to note: I think my original comment only affects goal-oriented error estimation. Having meshes with different cell types should still work for Hessian-based adaptation - I think :)

Edit: it will work if we don't pass finite_element to Field, since that will essentially tell the Field object what cell type to expect.