Implement the FSFT (Fast Spherical Fourier Transform), which is the equidistant version of the NFSOFT. Inside the NFSOFT routines, we replace the call of the NFFT by the call of the FFTW. This reduces computation time and memory consumption for the spherical Fourier transform with equidistant nodes.

I implemented it in the branch feature/fsft. Unit tests are still to be added.