Function a_qsm_multtr_real 

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

real(A_qsm^H * s): masked complex -> full-volume real.