paint an agent trail as ONE pixel to allow for multiple agent
        trails to be seen in the same cell

def paint_agent_trail(self, y, x, val):
        """
        paint an agent trail as ONE pixel to allow for multiple agent
        trails to be seen in the same cell
        """
        for j in range(1,self.cell_height-1):
            for i in range(1,self.cell_width-1):
                self.img.put(self.agent_color(val), (x*self.cell_width+i, y*self.cell_height+j))