DoubleFlatField returns nonsense results when sigma > 0
sigma > 0, the projections means
dff undergoes a high pass filter with:
dff = dff - gaussian_filter(dff, sigma)
However, this step is performed before phase retrieval and therefore before
-log. So at this stage, radios are still "intensities" values (or ratio
I/I0). Operations should be multiplicative, not additive (negative values before
-log would be fatal).