Vertices and Edges in invalid coordaintes should be undefined

[JanPfenning]

Vertices and Edges that are created with coordinates for every combination of x and y in the bounds of 0 to the respective height and width.
Not all of these coordinate values are valid.
The Coordinates may be created nevertheless but there should be a static function checking if the Coordinate value results in a valid Edge/Vertex.
Edges and Vertices should only be creatable with Valid Coordinates, not with invalid ones.
Although invalid coordinates are currently needed for the adjacency calculations.