Fix Arc and add @deprecated javadoc for students to implement.

src/main/org/insa/graph/Arc.java

  * @see ArcBackward
  *
  */
-public interface Arc {
+public abstract class Arc {
 
     /**
      * @return Origin node of this arc.
      */
-    public Node getOrigin();
+    public abstract Node getOrigin();
 
     /**
      * @return Destination node of this arc.
      */
-    public Node getDestination();
+    public abstract Node getDestination();
 
     /**
      * @return Length of this arc, in meters.
      */
-    public float getLength();
+    public abstract double getLength();
+
+    /**
+     * Compute the time required to travel this arc if moving at the given speed.
+     * 
+     * @param speed Speed to compute the travel time.
+     * 
+     * @return Time (in seconds) required to travel this arc at the given speed (in
+     *         kilometers-per-hour).
+     */
+    public double getTravelTime(double speed) {
+        return getLength() * 3600.0 / (speed * 1000.0);
+    }
 
     /**
      * @return Minimum time required to travel this arc, in seconds.
+     * 
+     * @see Arc#getTravelTime(double)
      */
-    public double getMinimumTravelTime();
+    public double getMinimumTravelTime() {
+        return getTravelTime(getRoadInformation().getMaximumSpeed());
+    }
 
     /**
      * @return Road information for this arc.
      */
-    public RoadInformation getRoadInformation();
+    public abstract RoadInformation getRoadInformation();
 
     /**
      * @return Points representing segments of this arc.
      */
-    public List<Point> getPoints();
+    public abstract List<Point> getPoints();
 }

src/main/org/insa/graph/ArcBackward.java

  * the original arc.
  *
  */
-class ArcBackward implements Arc {
+class ArcBackward extends Arc {
 
     // Original arc
     private final ArcForward originalArc;
     }
 
     @Override
-    public float getLength() {
+    public double getLength() {
         return this.originalArc.getLength();
     }
 
     @Override
-    public double getMinimumTravelTime() {
-        return this.originalArc.getMinimumTravelTime();
-    }
-
-    @Override
     public RoadInformation getRoadInformation() {
         return this.originalArc.getRoadInformation();
     }

src/main/org/insa/graph/ArcForward.java

  * arc implementation that stores data relative to the arc.
  *
  */
-class ArcForward implements Arc {
+class ArcForward extends Arc {
 
     // Destination node.
     private final Node origin, destination;
     }
 
     @Override
-    public float getLength() {
+    public double getLength() {
         return length;
     }
 
     @Override
-    public double getMinimumTravelTime() {
-        return getLength() * 3600.0 / (info.getMaximumSpeed() * 1000.0);
-    }
-
-    @Override
     public RoadInformation getRoadInformation() {
         return info;
     }