Topology-aware operators are incorrect at N+1?

[glwagner]

The change in this PR illustrates what would be done if this were correct.

cc @simone-silvestri

Or do we even want

@inline δxTᶠᵃᵃ(i, j, k, grid::AGXB{FT}, f, args...) where FT = ifelse((i <= 1) | (i >= grid.Nx+1), zero(FT), δxᶠᵃᵃ(i, j, k, grid, f, args...))

[simone-silvestri]

Ah I guess they are incorrect for i > N, I don't think they are ever used for that specific case, but good to correct it if some method wants to use them for i > N. I think the same should then apply to i == 0 for the center operators