Unable to decompose cat states

[ColbyDeLisle]

I added the following test to decompose.rs, attempting to check the decomposition of |cat_6> found on pg. 6 of this paper:

#[test]
fn cat6() {
    let mut g = Graph::new();

    let mut outputs = vec![];
    let z = g.add_vertex(VType::Z);
    for _ in 0..6 {
        let t = g.add_vertex_with_phase(VType::Z, Rational64::new(1, 4));
        g.add_edge_with_type(z, t, EType::H);

        let out = g.add_vertex(VType::B);
        g.add_edge(t, out);
        outputs.push(out);
    }
    g.set_outputs(outputs);

    let mut d = Decomposer::new(&g);
    d.with_full_simp().save(true).use_cats(true).decomp_all();  // this line panics

    assert_eq!(d.done.len(), 3);
}

Unfortunately the test never makes it to the assert_eq!; instead it panics on the line just prior with the error message "thread 'decompose::tests::cat6' panicked at quizx\src\graph.rs:559:22: Parallel edges only supported between Z and X vertices"

The problem persists when I change 6 to 5. When I change it to 4 the test still doesn't succeed, but instead of panicking it just hangs until I kill the process.

I'm hoping someone can verify that I haven't mistranslated the paper, and maybe has some inkling of where the issue lies. As with the other issue I opened, I'm more than happy to implement a fix if we can arrive at a diagnosis.

(I found this issue while trying to write some unit tests for the Python wrapper. Those unit tests are available on my branch here; I can make a PR to add those tests as well if it would be helpful.)