Spurious memory allocation reproduced with `poisson_dev_fe.jl`

This issue is to document a strange behaviour which is reproduced in `poisson_dev_fe.jl` when the following code is executed:

```
@time begin
for i in 1:100_000
smart_sum(manual_uₕ_array_at_qₖ)
end
end
```

A comment was attached to this code, that I move from there and put here to not forget

```
WHY THE SECOND PIECE OF CODE REQUIRES A NUMBER OF ALLOCATIONS THAT GROWS
WITH THE NUMBER OF CELLS? I CAN UNDERSTAND THAT THE CACHE ARRAY of
`manual_uₕ_array_at_qₖ` REQUIRES MORE MEMORY IN ABSOLUTE TERMS, BUT I AM NOT
ABLE TO SEE WHY IT GROWS WITH THE NUMBER OF CELLS!!!! ANY HINT?
```