Add multiple rays per detector option

Tom Pfeiffer commented at fully3D that one ray per detector may be little in some cases.

Most of the times I had simpy made a bigger detector via interpolation/geometry fiddling before using TIGRE, but I think it would make sense to remove this from the user and add it as a geo parameter (e.g. `geo.rays_per_pixel`), and let TIGRE handle this.

It should be "fairly" easy to do it in the entry points of CUDA, either in MATLAB/python Ax/Atb or actually in CUDA (probably better for speed)