Function a_qsm_mult 

fn a_qsm_mult(
    dip: &mut DipoleOp,
    x_img: &[f64],
    der_1st: &[Complex64],
    weight_vect: &[f64],
    mut_cst: f64,
) -> Vec<Complex64> 

A_qsm forward: image -> masked complex measurement.