code: def paint_agent_trail(self, y, x, val):
          
          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))
docs: paint an agent trail as ONE pixel to allow for multiple agent
          trails to be seen in the same cell