Macro graph 

macro_rules! graph {
    ($i:ident $op:tt $($tail:tt)*) => { ... };
    ($i:ident$(,)?) => { ... };
}

Constructs a Graph using an intuitive shorthand for node ordering relationships.

Example:

fn one(_: ()) -> Result<(), GraphError> {
    log::trace!("one");
    Ok(())
}
fn two(mut an_f32: ViewMut<f32>) -> Result<(), GraphError> {
    log::trace!("two");
    *an_f32 += 1.0;
    Ok(())
}
fn three(_: ()) -> Result<(), GraphError> {
    log::trace!("three");
    Ok(())
}

let _a = graph!(one < two, three, three > two);
let _b = graph!(one, two);
let _c = graph!(one < two);
let _d = graph!(one);
let _e = graph!(one < two < three);

let mut g = graph!(one < two < three).with_resource(0.0f32);
g.reschedule().unwrap();
let schedule = g.get_schedule();
assert_eq!(vec![vec!["one"], vec!["two"], vec!["three"]], schedule);