rename tilt_axis -> tilt_axis_rotation in RecParams
replace detector_shift_px by tilt_axis_par_shift_px in RecParams

Compute the parallel shift of the tilt axis and always assume that
the projection images are centered in 0.
use tilt axis shift instead of detector shift

There's still a need to think about how to properly handle a shifted
tilt axis. The current version seems to work, but further tests are
required.
remove detector shift, add tilt axis shift
Properly account for axis shifts in single-axis backprojection
Remove z axis issue from TODO