A grid graph implementation

Extend previous branch with GridGraph, a grid graph implementation, supporting O(1) access and a random access iterator interface.
I had made a mistake actually overlooking this. This is now:
```
) const {
    assert(j < numberOfEdgesToVertex(vertex));
    VertexCoordinate vertexCoordinate;
    this->vertex(vertex, vertexCoordinate);
    size_type direction;
    bool isSmaller;
    return vertexFromVertex(vertexCoordinate, j, direction, isSmaller);
}
```