no nested links

Closes #9 (closed)

No more nested "links" attributes

This

{
 "links: [
    {
        "source": "node1",
        "target": "node2",
        "links": {
            "sub_target": "subnode1",
            "links": {"sub_target": "subsubnode1"},
        },
    }
 ]
}

is replaced by this

{
 "links: [
    {
        "source": "node1",
        "target": "node2",
        "sub_graph_nodes": {
            "sub_target": "out",
        },
    }
 ]
}

Graph input and output

This is introduced so that sub_graph_nodes doesn't need to be nested

    {
        "graph": {
            "input_nodes": [
                {"alias": "in1", "id": "name1"},
                {"alias": "in2", "id": "name2", "sub_node": "name4"},
            ],
            "output_nodes": [
                {"alias": "out1", "id": "name5"},
                {"alias": "out2", "id": "name6", "sub_node": "name7"},
            ]
        }
    }