Add check to ensure complex alignment

[yasahi-hpc]

Resolves #226

In kokkos-fft, we strongly assume the 32 bit alignment of Kokkos::complex type.
This PR checks whether KOKKOS_ENABLE_COMPLEX_ALIGN is enabled in CMake and cpp code.

[tpadioleau]

Seems good to me. Could you check that the detection works if the option is missing ?

[yasahi-hpc]

For CUDA, I got the following errors if we compile with -DKokkos_ENABLE_COMPLEX_ALIGN=OFF

kokkos-fft/fft/src/KokkosFFT_default_types.hpp(11): error: static assertion failed with "KokkosFFT requires option -DKokkos_ENABLE_COMPLEX_ALIGN=ON to build Kokkos"

kokkos-fft/fft/src/KokkosFFT_Cuda_types.hpp(20): error: static assertion failed

kokkos-fft/fft/src/KokkosFFT_Cuda_types.hpp(23): error: static assertion failed

I think we need to improve the error messages under KokkosFFT_Cuda_types.hpp. I will do that in another PR

[tpadioleau]

For CUDA, I got the following errors if we compile with -DKokkos_ENABLE_COMPLEX_ALIGN=OFF

kokkos-fft/fft/src/KokkosFFT_default_types.hpp(11): error: static assertion failed with "KokkosFFT requires option -DKokkos_ENABLE_COMPLEX_ALIGN=ON to build Kokkos"

kokkos-fft/fft/src/KokkosFFT_Cuda_types.hpp(20): error: static assertion failed

kokkos-fft/fft/src/KokkosFFT_Cuda_types.hpp(23): error: static assertion failed

Seems good, what about the cmake detection ?

[yasahi-hpc]

If we rely on installed Kokkos with -DKokkos_ENABLE_COMPLEX_ALIGN=OFF

-- Could NOT find Kokkos_OPTIONS (missing: COMPLEX_ALIGN)
CMake Error at lib/kokkos4.5_unalign/lib64/cmake/Kokkos/KokkosConfigCommon.cmake:112 (message):
  Kokkos does NOT provide all backends and/or architectures requested
Call Stack (most recent call first):
  CMakeLists.txt:42 (kokkos_check)

Should be OK

[tpadioleau]

Ok for me!

[yasahi-hpc]

@tpadioleau
Thank for your review. I will merge this