Function rest::dft::prepare_tabulated_sigma

fn prepare_tabulated_sigma(
    rhop: &RIFull<f64>,
    spin_channel: usize
) -> MatrixFull<f64>

Prepare sigma[0] = rhop_u dot rhop_u => sigma_uu sigma[1] = rhop_u dot rhop_d => sigma_ud sigma[2] = rhop_d dot rhop_d => sigma_dd