Function write_solution 

pub fn write_solution<'a, W, G, T, Fs>(
    w: W,
    g: &'a G,
    flow: Fs,
    value: T,
) -> Result<()>
where
    W: Write,
    G: IndexDigraph,
    T: Display + Zero,
    Fs: Fn(G::Edge<'a>) -> T,

Write a solution of a min-cost-flow problem.