Function susceptibility_artifacts_step 

fn susceptibility_artifacts_step(
    x0: &[f64],
    xfs: &[f64],
    mask: &[u8],
    d: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
    tol: f64,
    maxit: usize,
    _workspace: &mut Fft3dWorkspace,
) -> Vec<f64>

Step 3: Estimate streaking artifacts using LSMR