Remove the same node id check in network

[wenzeslaus]

The test for "Edge cannot begin and end with the same node" should not be there. Loop is not an error and with dropping dispersers between nodes, it yields non-trivial results.

            if (node_1_id == node_2_id) {
                throw std::runtime_error(
                    std::string("Edge cannot begin and end with the same node: ")
                    + node_1_text + " " + node_2_text);
            }