Function fastqsm_step 

fn fastqsm_step(
    f: &[f64],
    mask: &[u8],
    d: &[f64],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f64,
    vsy: f64,
    vsz: f64,
    workspace: &mut Fft3dWorkspace,
) -> Vec<f64>

Step 2: FastQSM estimate