Add Named-Port Network Builder

The package already supports the core SLH reduction rules: cascade, concatenate, and feedback. However, complicated networks still require manual port bookkeeping with integer indices, e.g.

feedback(G, 1 => 3, 3 => 5, 5 => 7)

This is powerful but easy to get wrong. A higher-level named-port interface would make large networks easier to read, debug, and teach.

net = Network()

add!(net, :source, coherent_drive(α), outputs=[:out])
add!(net, :cavity, one_sided_cavity(a; κ, Δ), inputs=[:in], outputs=[:out])
add!(net, :phase, phase_shifter(ϕ), inputs=[:in], outputs=[:out])

connect!(net, :source => :out, :cavity => :in)
connect!(net, :cavity => :out, :phase => :in)

G_eff = reduce(net)

And then we can add plotting

plot_network(net)
plot_reduction_steps(net)

Start by locating the existing cascade, concatenate, and feedback entry points that implement the current SLH reductions. Define the scope of Network, add!, connect!, reduce, and the proposed plotting functions; done means named connections can express the example network and reduce through the existing rules, with plotting scope clarified.