Function axial_grid_for 

pub fn axial_grid_for(
    nx: usize,
    ny: usize,
    nz: usize,
    affine: &[f64; 16],
) -> AxialGrid

Build the axial grid covering the same world-space extent as an oblique volume, keeping its voxel sizes. The new affine is diagonal, so b0_direction_from_affine on it returns (0, 0, 1).