Remove multi-slice logic in backprojector

The multi-slice backprojector (BACKPROJ_3D) was written to see if we could gain some speed by backprojecting multiple slice at once (instead of looping over slices). It turned out it was not a good idea.

Besides, some code paths in filtering.py do not take into account the multi-slice (eg. when padding="edges).

The multi-slice code should be removed from backproj.cu, fbp.py and filtering.py.