Use fft-based smoothing filter for object smoothing in GPU-accelerated projection engines

[daurer]

can re-use FFTGaussianSmoothingKernel implemented for ML_pycuda and ML_cupy.

[jcesardasilva]

Hi, I have finally checked it, and it seems to be working well now. The old problem seems to no longer exist. The new exposed parameter smooth_gradient_method is a very good asset. I will keep testing it in the long term, but all seems good to me. Thank you!

Nevertheless, I have only tested with ML_pycuda since I have not yet been successful in installing CuPy in order to try ML_cupy, but I assume it should work as well. I have the same installation problem in the 2 HPC clusters I have access to.