//Makes an edge from x_par to x_chl
//Updates x_chl's cost by the sum of x_par's cost and the distance between x_par and x_chl
private static void AddEdge(Sample x_par, Sample x_chl)
{
x_par.AddChild(x_chl);
x_chl.parent = x_par;
float dist = (x_par.pos - x_chl.pos).magnitude;
x_chl.cost = x_par.cost + dist;
}