handle mouse wheel zoom changes

def on_mouse_wheel(self, event):
        '''handle mouse wheel zoom changes'''
        rotation = event.GetWheelRotation() / event.GetWheelDelta()
        if rotation > 0:
            zoom = 1.0/(1.1 * rotation)
        elif rotation < 0:
            zoom = 1.1 * (-rotation)
        self.change_zoom(zoom)
        self.redraw_map()