oct-unfold FFT warning
Description
Call fft_all_end after destroying the electronic system, as this one destroys the Poisson FFTs and else, this leads to a warning using the oct-unfold utility.
News snippet
Fix an order of destroy operations to avoid a warning.
Checklist
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I have checked that my code follows the Octopus coding standards -
I have added tests for all the new features added in this request.