finds a place on the grid which is clear on all sides 
        to avoid starting in the middle of a blockage

def find_safe_starting_point(self):
        """
        finds a place on the grid which is clear on all sides 
        to avoid starting in the middle of a blockage
        """
        y = random.randint(2,self.grid_height-4)
        x = random.randint(2,self.grid_width-4)
        return y, x