Switch to scipy.irfftn

This merge request consist of six commits:

Time and memory usage of test_time_and_memory (8192x64x64) with and without high frequency compensation

id	time [s]	memory [MB]	max abs diff hfc=0	max abs diff hfc=1	description
Master	33.9	6405
pre_irfft_updates	35.6	6918	0	0	Move scaling and vectorize. Changes without impact on result (slightly slower and more memory consuming - this will change when switching to from numpy.fft to scipy.fft)
scipy_fft	30.2	4998	2.85e-06	3.13e-06	Switch from numpy.fft to scipy.fft which allows single precision and parallel execution
reverse_fft_order	27.1	4991	3.81e-06	3.81e-06	reverse fft order such that real fft can be made on x dimension which is typically the larger. Furthermore, cropping in x dimension is simpler.
crop_spectral_tensor	17.2	2709	2.86e-06	2.86e-06	Switch to irfft and crop unnecessary part of spectral tensor, i.e. reduce from (3,3,Nx,Ny,Nz) to (3,3,Nx//2+1,Ny,Nz).
crop_random_numbers	15.8	2709	13.21	14.08	Only generate necessary random numbers. This change results a completely different box due to different random numbers
complex64 to float32	15.3	2114	0	0	Store SqrtPhi as float32 instead of complex64 until multiplied with complex numbers