Reuse plan

[yasahi-hpc]

In this PR, I have added the capability to reuse a plan for the same FFTs. This is particularly important for NVIDIA GPUs, which has some overheads to create plans.

Now the following operations are possible.

int axis = -1;
KokkosFFT::Impl::Plan fft_plan(execution_space(), xc2c, xc2c_hat, KokkosFFT::Impl::Direction::Forward, axis);
KokkosFFT::fft(execution_space(), xc2c, xc2c_hat, fft_plan);