Use symmetry of copying in crossing minimization in `to_hom_expr`

I'm using the following code to generate a TikZ diagram:

```julia
using Catlab, Catlab.Theories
using Catlab.Programs
using Catlab.WiringDiagrams
using Catlab.Graphics
import TikzPictures, Convex, SCS

@present P(FreeSymmetricMonoidalCategory) begin
(M,S,H)::Ob
psi::Hom(M,H)
kappa::Hom(H,otimes(H,S))
u::Hom(otimes(S,M), M)
end

# hack to make the greek letters render correctly
P[:psi].args[1] = Symbol("\\psi")
P[:kappa].args[1] = Symbol("\\kappa")

diagram = @program P (m1::M) begin
h1 = psi(m1)
h2, s = kappa(h1)
m2 = u(s, m1)
h3 = psi(m2)
return h3, m2
end

tikz_diag = to_tikz(
to_hom_expr(FreeCartesianCategory,diagram),
labels=true,
)
```

The diagram comes out looking like this:

Why is that swap there after the second copy morphism, and is there a way to remove it? (Or better still, have it removed automatically.)