Axis enum cleanup

1. Don't increment Axis values
   • Loop with range-for syntax using initialization lists
   • Remove the incr (Axis) function
   • Use other_axis (Axis) in more places
2. Change the type of NO_AXES from Axis to size_t. Replace Side_position_interface::get_axis with is_on_x_axis and is_on_y_axis instead of returning an Axis set to NO_AXES.
3. Align_interface: convert SCM to Axis robustly. X_AXIS was chosen as the default just because it has the value 0.
4. Use from_scm<Axis> more widely. These places seemed to be coded robustly already, but consistency is good.