on powerpc, fft(zeros) sometimes gives phase = pi

[Paul Brossier]

when computing the fft on a vector of zeroes, the phase elements can be found equal to zero or pi on powerpc, whereas they are always zero on x86.

this is due to the way real and imaginary parts are computed in fftw on both platforms. on x86. the result is always +0.0000, whereas on ppc, it can be -0.0000; atan2f(-0.00,0.00) = pi

see test in source:tests/python/src/spectral/fft.py, test_do_zeroes

[Paul Brossier <piem@piem.org>]

(In ,543) closes #4, fft.c: if real part is zero, have phase = 0