Initial graph class implementation

[MathyouMB]

Please implement the graph class on this file.

Graph should have be initialized with edges and vertices which will also need to be implemented.

There should be a methods for deleting and adding vertices and deleting and adding edges. If you delete a vertex, you need to delete it's associated edges.

 let graph = new Graph(nodes, links);

  graph.addNode()
  graph.addEdge()