get rid of MakeTopology()

[rade]

func MakeTopology() Topology {
    return Topology{
        Nodes:    map[string]Node{},
        Controls: Controls{},
    }
}

We should just be able to write Topology{} at all the call sites. There is nothing intrinsically special about Nodes and Controls compared to all the other topology fields, though @paulbellamy thinks that they may not have valid nil values (yet).