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": {}
		}
	]
}