def select(&block)
      return self.to_enum(:select) unless block_given?
      rows = []
      i = 0
      self.each_row do |row|
        if row.instance_eval(&block)
          rows.push(row)
        end
      end
      Mikon::DataFrame.new(rows)
    end
def select(&block)
      return self.to_enum(:select) unless block_given?
      rows = []
      i = 0
      self.each_row do |row|
        if row.instance_eval(&block)
          rows.push(row)
        end
      end
      Mikon::DataFrame.new(rows)
    end
Select rows using Mikon::Row DSL and create new DataFrame
 @example
   df = Mikon::DataFrame.new({a: [1,2,3], b: [2,3,4]})
   df.select{a%2==0}[:a].to_a #-> [2]