Use Iterators.flatten for neighbors

[amontoison]

In this function, we could use Iterators.flatten if we we know the index of the diagonal coefficients.

for i in Iterators.flatten((1:2, 4:5))
  ...
end

[gdalle]

I seem to remember having tried that, and concluded that it wasn't faster. But it would deserve some more benchmarking

[amontoison]

We don't need it anymore with the strategy of #214 because we also need to iterate over the self-loop.