addEdge abstract method

Adds an edge from source to target with a weight to the graph.

Returns the previous weight of the edge, or null if the edge was not present.

If the edge was added, the graph must:

• Add the edge to the graph so that containsEdge returns true;
• Yield target as a successor of source in successors;
• If source has no predecessors, return true for containsRoot;

In addition, the graph may, depending on the implementation:

• Add source and target to the graph if they are not already present;
• Consider the inverse also an edge, i.e. for undirected graphs.