Implement General separable footprint projectors

[AnderBiguri]

Related #121

Tim Pfeiffer at Fully3D presented a general Separable Footprint projection (Poster 03-02 - Two extensions of the separable footprint forward projector).

I believe this are still not applicable to all geometry, but it could be interesting to have it implemented.

Its probably at https://gitlab.com/tpfeiffe/ctl. I believe its MIT, which shoudl be compatible with BSD, but I'd need to double check.