Path class

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Create a class named Path in a new tab named Path
The Path class handles the link between two points, A and B.

Fields:   iA, iB   // index to each node

Constructor:   Path( int i_A, int i_B) {...}
    Simply copy arguments to fields.

Function:
    void draw() {
        int xA = ((Node)nodes.get(iA).x0;
        ...
        draw a black line connecting (xA, yA) and (xB, yB)
    } //draw