Add GML object format

[Elscrux]

This adds GML as the output format for MaxCut solvers using a new object format of GML.

[schweikart]

When I enter the following demo problem, the labels are converted to strings and double-quoted. I don't think we can avoid the conversion to strings but the double-quote should not be necessary:

graph [
    id 42
    node [
        id 1
        label "1"
    ]
    node [
        id 2
        label "2"
    ]
    node [
        id 3
        label "3"
    ]
    edge [
        source 1
        target 2
    ]
    edge [
        source 2
        target 3
    ]
    edge [
        source 3
        target 1
    ]
]

Result:

{
    "nodes": [
        {
            "id": 1,
            "label": "\"1\"",
            "attributes": {
                "partition": "1"
            }
        },
        {
            "id": 2,
            "label": "\"2\"",
            "attributes": {
                "partition": "0"
            }
        },
        {
            "id": 3,
            "label": "\"3\"",
            "attributes": {
                "partition": "1"
            }
        }
    ],
    "edges": [
        {
            "source": 1,
            "target": 2,
            "attributes": {}
        },
        {
            "source": 2,
            "target": 3,
            "attributes": {}
        },
        {
            "source": 3,
            "target": 1,
            "attributes": {}
        }
    ]
}