Avoid the creation of a temporary array when this is not needed.
Description
Avoid the creation of a temporary array when this is not needed. This temporary array can make the routine fft_forward twice more expensive than it should.
News snippet
Avoid the creation of a temporary array when this is not needed.
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.