Completely remove inverse smoothing
The inverse_times() of both
SmoothingOerators is currently broken.
DirectSmoothingOperator considers the inverse operation of "smoothing with sigma" to be "smoothing with 1/sigma", which is simply wrong.
FFTSmoothingOperator does in principle perform the correct operations, but does not check for division by zero, which occurs for most sigma values and indicates that the smoothing is in fact non-invertible with finite precision arithmetics.
Unfortunately the test
test_inverse_adjoint_times does not recognize this, because the results it compares are both NaN and therefore the same...
Since this operation appears to be unused, I'd suggest to remove it ... doing it properly may be possible (at least for FFT smoothing), but is only worth the effort if someone really needs it.