problem with incremental interpolating backprojector

[KrisThielemans]

On some systems/compilers, this projector has trouble in the central pixel of each plane, and sometimes on pixels on the 45 degrees (and related) angles. This is due to (unavoidable?) rounding errors that are a consequence of the incremental scheme (voxel location and voxel value are essentially decoupled). The errors tend to disappear in debug mode, so have to do with optimisation in math calculations.

This is a very old bug. It'd probably be best to get rid of this projector and replace it with a non-incremental interpolating one (vastly simpler code as well).