Update TODO
Update version
Introduce new tilt axis determination

The RecParams structure contains a new 'tilt_axis' member.
If the tilt_axis parameter in the config file is between -45 and +45 degrees,
tilt_axis is set to 0 (-> x axis tilt) and 'tilt_axis_rotation' is set to the
config file parameter.
Otherwise, tilt_axis is set to 1 (-> y axis tilt), and 'tilt_axis_rotation'
is calculated as the config file parameter minus (-)90 degrees according to
the sign of the parameter.
This way, image_rotation will automatically align the tilt axis with either
the x or y axis, and the correct backprojection function will be called.
Fix bad rotation vector definition
Add generic single-axis and 'say' backprojections

- 'say' is the fast variant of the single-axis-y backprojection
- Instead of calling these directly, the call is wrapped into a
generic function deciding about the variant according to the
tilt_axis value in the RecParams structure
Make single-axis reco kernel work for y axis tilt

Upon init, the single-axis reco kernel is computed depending on the
tilt_axis parameter in the RecParams structure.
Use generic single-axis functions