@Override
    public void write(JmeExporter ex) throws IOException {
        OutputCapsule oc = ex.getCapsule(this);
        oc.write(spline, "spline", null);
    }
interpolate the path giving the time since the beginning and the motionControl
this methods sets the new localTranslation to the spatial of the MotionEvent control.
@param time the time since the animation started
@param control the control over the moving spatial
@param tpf time per frame (in seconds)
@return the distance travelled (in world units)
jme3-core/src/main/java/com/jme3/cinematic/MotionPath.java