Iterate over a node list instead of using nested loops.

[ali-ramadhan]

@peterahrens suggested iterating over a list of nodes, e.g. using a CartesianIndices iterator instead of looping over all the indices with a nested for loop.

However, according to @vchuravy this may not work with GPUifyLoops.jl right now so this issue has no associated milestone. Actually, see: vchuravy/GPUifyLoops.jl#18 (comment)

This will actually be really great once we move to other non-Cartesian grids as we can just use a different iterator (CubedSphereIndices?) and not have to modify the time stepping code.

See: Multidimensional algorithms and iteration

[glwagner]

See

https://github.com/glwagner/StencilLab.jl/blob/ca1e9b2f0b32a48cfcd777d1519f7faa1ed14d02/src/kernels.jl#L19

for one way to use a CartesianIndex.

[ali-ramadhan]

I think we'll end up doing this when we switch to KernelAbstractions.jl anyways so I'm closing this issue as it has become stale.