Bug in ray-tracing projector

[markus-jehl]

The ray-tracing projector creates small artefacts when configured to use more than 1 tangential LORs. To reproduce the issue, run the test BlocksTests::run_symmetry_test() in recon_test/test_blocks_on_cylindrical_projectors.cxx and look at the sino1_from_image.hs, then add the line "PM->set_num_tangential_LORs(5);" to the definition of the projection matrix and re-run.