first pass to fix all spectral operator with the new backend handling

hysop/numerics/fft/fft.py

hysop/numerics/fft/host_fft.py

hysop/numerics/fft/opencl_fft.py

hysop/operator/base/curl.py

hysop/operator/base/solenoidal_projection.py

hysop/operator/base/spectral_operator.py