FFT on Host and Device

[yasahi-hpc]

In this PR, I have added the capability to call KokkosFFT APIs from both host and device. This is not quite useful so I will make it optional. (-DKokkosFFT_ENABLE_HOST_AND_DEVICE option)
If you enable this option for CUDA backend, you need fftw in addition to cufft.

Now the following operations are possible.

using execution_space = Kokkos::DefaultExecutionSpace;
using host_execution_space = Kokkos::DefaultHostExecutionSpace;
KokkosFFT::fft(execution_space(), a, out); // Calling FFTs on Device
KokkosFFT::fft(host_execution_space(), h_a, h_out); // Calling FFTs on Host