Add extension method to get directed edge id on routerpoint.

[xivk]

Add this to RouterPointExtensions:

        /// <summary>
        /// Gets the directed edge id for the routerpoint.
        /// </summary>
        public static long EdgeIdDirected(this RouterPoint point, bool forward = true)
        {
            if (forward)
            {
                return point.EdgeId + 1;
            }
            return -(point.EdgeId + 1);
        }