NFFT and compiler warning

Activating the -Wconversion for the C/C++ compiler seem to reveal a potential bug. We should probably investigate it.

/scratch/cioctopu/static/9082438815/octopus-code/octopus/src/math/nfft_f.c:302:12: warning: conversion from 'fftw_complex' {aka '_Complex double'} to 'double' discards imaginary component [-Wconversion]
/scratch/cioctopu/static/9082438815/octopus-code/octopus/src/math/nfft_f.c:305:12: warning: conversion from 'fftw_complex' {aka '_Complex double'} to 'double' discards imaginary component [-Wconversion]
/scratch/cioctopu/static/9082438815/octopus-code/octopus/src/math/nfft_f.c:308:12: warning: conversion from 'fftw_complex' {aka '_Complex double'} to 'double' discards imaginary component [-Wconversion]
/scratch/cioctopu/static/9082438815/octopus-code/octopus/src/math/nfft_f.c:352:12: warning: conversion from 'fftw_complex' {aka '_Complex double'} to 'double' discards imaginary component [-Wconversion]
/scratch/cioctopu/static/9082438815/octopus-code/octopus/src/math/nfft_f.c:355:12: warning: conversion from 'fftw_complex' {aka '_Complex double'} to 'double' discards imaginary component [-Wconversion]
/scratch/cioctopu/static/9082438815/octopus-code/octopus/src/math/nfft_f.c:358:12: warning: conversion from 'fftw_complex' {aka '_Complex double'} to 'double' discards imaginary component [-Wconversion]