Let TopologyBuilder handle 2-to-n transition

[redeboer]

There seems to be a bug in the SimpleStateTransitionTopologyBuilder: it doesn't correctly handle a list of InteractionNodes:

from expertsystem.reaction.topology import (
    InteractionNode,
    SimpleStateTransitionTopologyBuilder,
)

int_nodes = [
    InteractionNode("TwoBodyDecay", 1, 2),
    InteractionNode("TwoBodyProduction", 2, 1),
]
topology_builder = SimpleStateTransitionTopologyBuilder(int_nodes)
topology_builder.build_graphs(2, 3)

    149             connected_nodes = edge.get_connected_nodes()
    150             if not connected_nodes:
--> 151                 raise ValueError(
    152                     f"Edge nr. {edge_id} is not connected to any node ({edge})"
    153                 )

ValueError: Edge nr. 1 is not connected to any node (Edge(ending_node_id=None, originating_node_id=None))

If I understand correctly, the above snippet should handle a 2-to-3 transition that should look like this:

Code to create the figure

import graphviz

import expertsystem as es
from expertsystem.reaction.topology import Edge, Topology

topology = Topology(
    nodes={0, 1, 2},
    edges={
        0: Edge(0, None),
        1: Edge(0, None),
        2: Edge(1, 0),
        3: Edge(2, 1),
        4: Edge(None, 1),
        5: Edge(None, 2),
        6: Edge(None, 2),
    },
)
dot = es.io.convert_to_dot(topology)
graphviz.Source(dot)