Build fails with concurrent feature

[Nashtare]

PR #124 broke the build using concurrent feature, by changing the scope of the fft_in_place function (originally in fft/serial module) to private, and moving it to the fft/fft_inputs module without updating the calls in the fft/concurrent module.

I think this already happened in the past that a change breaking builds with parallelization went unnoticed in the main branch, perhaps we should consider having a check in the CI?