issue #17 fixed in xray_backprojection_sax
Fix bad assignment in RecParams_apply_to_proj_image

Instead of detector_shift_px, vol_shift_px was used to compute the
new image origin.
Final fix for issue #17

Both xray_backprojection and xray_backprojection_sax now implement
the rotations including shifts of the tilt axis and of the
volume correctly.
Still needs some testing, though.
include commented-out code for generic backprojection