Resolve "rename sub-graph links"

Closes #7 (closed)

Deprecated way to specify submodel links (still supported until MX workflows are converted)

link_attrs = {
    "source": "subgraph1",
    "target": "subgraph2",
    "links": [
        {
            "source": "subsubgraph",
            "target": "task1",
            "links": {
                "source": "subsubsubgraph",
                "links": {"source": "task2", "arguments": {0: "return_value"}},
            },
        }
    ],
}

New way to specify submodel links

link_attrs = {
    "source": "subgraph1",
    "target": "subgraph2",
    "arguments": {0: "return_value"},
    "sub_graph_nodes": {
        "sub_source": "subsubgraph",
        "sub_target": "task1",
        "sub_graph_nodes": {
            "sub_source": "subsubsubgraph",
            "sub_graph_nodes": {"sub_source": "task2"},
        },
    },
}