def to_ru
      m=Matrix.build(self.row_size, self.column_size){nil}
      self.each_with_index do |x, row, col|
        if x.nil? || x.value.nil?
          m[row, col] = nil
        else
          m[row, col] = x.to_ru
        end
      end
      return m
    end
def to_ru
      m=Matrix.build(self.row_size, self.column_size){nil}
      self.each_with_index do |x, row, col|
        if x.nil? || x.value.nil?
          m[row, col] = nil
        else
          m[row, col] = x.to_ru
        end
      end
      return m
    end
The to_ru method returns a Matrix of "rubified" values. It basically builds a new Matrix
 and puts the result of the #Cell::to_ru method of every cell of the original sheet in
 the corresponding Matrix cell.
 @return [Matrix] a collection of ruby objects (#Integers, #Floats, #DateTimes, #Rationals, #Strings)