FFTOperator fails for zerocenter=True spaces.
Both scenarios fail:
from nifty import *
x = RGSpace((16,), zerocenter=True)
f = Field((x,x), val=1)
fft = FFTOperator(x)
fft(f, spaces=(1,))from nifty import *
x = RGSpace((16,), zerocenter=True)
f = Field((x,), val=1)
fft = FFTOperator(x)
fft(f)