Possible cleanup needed in cuda complex reorder kernels

[JamieJQuinn]

Currently, the subroutines in src/cuda/kernels/complex.f90 are considered temporary and tied to the current implementation of the FFT Poisson solver using cuFFT. When this is reimplemented in cuFFTMP, these functions are likely to be removed.

Only if we decide to keep the existing reordering subroutines specific to cuFFT should we tidy those subroutines up and document properly. This issue is just a reminder and can be closed if the relevant subroutines are removed as planned.