def each
      (1..ROWS).each do |r|
        (1..COLS).each do |c|
          yield(r, c, self[r, c])
        end
      end
    end
def each
      (1..ROWS).each do |r|
        (1..COLS).each do |c|
          yield(r, c, self[r, c])
        end
      end
    end
Iterates over all the positions of this grid from left to right and top to bottom.

 @example
  g = Grid.new
  g.each do |r, c, k|
    puts "(#{r}, #{c}) -> #{k}"
  end