Add continuous backprojection support

I think it's time to make things a little better optimized for Concert. Currently we reconstruct the *whole* sinogram from Concert. This adds the possibility to start at some angle offset and add the backprojected parts to the slice. However, the `n_projections` parameter cannot be figured out from the input anymore since only data portions will come there. I think we could make it a real parameter and also one like `offset` which we could change from concert all the time for a burst of new projections in the sinogram.
This changes the current backprojection API quite a lot, but I think there is no need to make it a new filter.