Better FFT interpolation for singular integration

[f0uriest]

FFT interpolation can't handle non-symmetric grids. We will probably always need the source grid to be symmetric to capture the integral correctly, but in theory the evaluation grid could be symmetric if we get the interpolation to work correctly.

Originally posted by @ddudt in https://github.com/PlasmaControl/DESC/pull/725#discussion_r1495256689

[dpanici]

can't handle symmetric grids right?