Replace SWFFT

[ajnonaka]

Replace SWFFT

[ajnonaka]

In https://github.com/ajnonaka/fft-test.git directory fft-test/local I have a serial implementation of fftw given input multifab data. I figured out the complex conjugate reflecting to make it match SWFFT (which needs to be shifted again to put k=0 at the center)

For domains from (0,0,0) to (Nx-1,Ny-1,Nz-1)

For any cells with i index >= Nx/2, these values are complex conjugates of the corresponding entry where (Nx-i,Ny-j,Nz-k) UNLESS that index is zero, in which case you use 0.

e.g. for an 8^3 domain, any cell with i index

Cell (6,2,3) is complex conjugate of (2,6,5)

Cell (4,1,0) is complex conjugate of (4,7,0) (note that the FFT is computed for 0 <= i <= Nx/2)

[ajnonaka]

I believe for all the structure factor FFTs (including flattened multifabs 3D->2D and 2D->1D) that serial FFTW has now been implemented. I have tested this for both dimensionalities, full and flattened multifabs, parallel runs with multiple grids on the regular domain, and non-cubic domains. Leaving this issue open until a few more tests have been run.