DoubleFlatField returns nonsense results when sigma > 0

When using DoubleFlatField with 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).