*nodes return reduced dimension arrays/views whereas *spacings returns KernelFunctionOperators

[ali-ramadhan]

Following the merging of #3143 the *spacings functions return a KernelFunctionOperator.

This is great, but should we be worried that this is inconsistent with the *nodes functions which return reduced dimension arrays/views?

If *nodes also returned KernelFunctionOperators would this make for a more consistent and powerful user interface?

---

julia> grid = RectilinearGrid(topology = (Periodic, Periodic, Bounded),
                              size = (4, 5, 4),
                              halo = (1, 1, 1),
                              x = (0, 1),
                              y = (0, 1),
                              z = [0, 0.1, 0.3, 0.6, 1])
4×5×4 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 1×1×1 halo
├── Periodic x ∈ [0.0, 1.0) regularly spaced with Δx=0.25
├── Periodic y ∈ [0.0, 1.0) regularly spaced with Δy=0.2
└── Bounded  z ∈ [0.0, 1.0] variably spaced with min(Δz)=0.1, max(Δz)=0.4

julia> znodes(grid, Face())
5-element view(OffsetArray(::Vector{Float64}, 0:6), 1:5) with eltype Float64:
 0.0
 0.1
 0.3
 0.6
 1.0

julia> zspacings(grid, Face())
KernelFunctionOperation at (⋅, ⋅, Face)
├── grid: 4×5×4 RectilinearGrid{Float64, Periodic, Periodic, Bounded} on CPU with 1×1×1 halo
├── kernel_function: zspacing (generic function with 27 methods)
└── arguments: ("Nothing", "Nothing", "Face")

[glwagner]

I think so, because then coordinates can participate in AbstractOperations.

I noticed that PartialCellBottom does not extend znode. But perhaps it should. In that case, KernelFunctionOperation is required.