Moves the turtle by distance in the direction it is facing. 
        If the pen is lowered it will also add to the currently drawn polyline.

def _move(self, distance):
        """ Moves the turtle by distance in the direction it is facing. 
        If the pen is lowered it will also add to the currently drawn polyline.
        """
        self._position = self._position + self._orient * distance  
        if self.isPenDown(): 
            x = round(self._position.x, 2)
            y = round(self._position.y, 2)
            self._pointsOfPolyline.append(Vector(x, y))