Move FFT routines to src/gromacs/fft/.

Adjusted the build system a bit to select the file to build at the CMake
level instead of using #ifdefs within the files. Some minor cleanup.
Also add some hacks to produce separate Doxygen docs for each of the
gmx_fft_t implementations; otherwise, they mess up each other and
produce warnings, as some of them have Doxygen comments and some do not.

Change-Id: I55d8c4ce402d68b8cd3e2e700f83d801d8b8294e