Function multi_port_nr_solve_into 

pub fn multi_port_nr_solve_into(
    n_nl: usize,
    s_nl: &[f64],
    known_a: &[f64],
    port_resistances: &[f64],
    devices: &[&dyn NlDeviceIv],
    v_guess: &mut [f64],
    max_iter: usize,
    tolerance: f64,
    ws: &mut NrWorkspace,
)

Like multi_port_nr_solve, but writes results into ws.b_nl to avoid allocation.