Rename: KokkosFFT_OpenMP*.hpp to KokkosFFT_Host*.hpp for fftw backend

[yasahi-hpc]

Resolves #110

Renaming files for fftw backend:

 rename fft/src/{KokkosFFT_OpenMP_plans.hpp => KokkosFFT_Host_plans.hpp} (98%)
 rename fft/src/{KokkosFFT_OpenMP_transform.hpp => KokkosFFT_Host_transform.hpp} (94%)
 rename fft/src/{KokkosFFT_OpenMP_types.hpp => KokkosFFT_Host_types.hpp} (97%)

[jbigot]

is it host or parallel-host? By that I mean: is there a separate set of files for host serial?

[cedricchevalier19]

@jbigot, I think it is from #109; I saw that the OpenMP files are also for Threads and Serial backends.

[jbigot]

I see, then it sounds good

[yasahi-hpc]

@jbigot

is it host or parallel-host? By that I mean: is there a separate set of files for host serial?

As @cedricchevalier19 said, this handles all host backends, so Host should be a better name.
In the current implementation, the only difference between parallel_host and host is the call of init_threads for fftw or not