def to_m(*attrs)
      m=Matrix.build(self.row_size, self.column_size){nil}
      self.each_with_index do |x, row, col|
        if attrs.size == 0 || attrs.nil?
          m[row, col] = x.value
        end
      end
      return m
    end
def to_m(*attrs)
      m=Matrix.build(self.row_size, self.column_size){nil}
      self.each_with_index do |x, row, col|
        if attrs.size == 0 || attrs.nil?
          m[row, col] = x.value
        end
      end
      return m
    end
The to_m method returns a simple Matrix object filled with the raw values of the original Sheet object.
 @return [Matrix] a collection of string values (the former #Cell::value)